diff --git a/openfisca_core/entities/__init__.py b/openfisca_core/entities/__init__.py
index 9546773cb..927aad63d 100644
--- a/openfisca_core/entities/__init__.py
+++ b/openfisca_core/entities/__init__.py
@@ -1,40 +1,94 @@
-# Transitional imports to ensure non-breaking changes.
-# Could be deprecated in the next major release.
-#
-# How imports are being used today:
-#
-#   >>> from openfisca_core.module import symbol
-#
-# The previous example provokes cyclic dependency problems
-# that prevent us from modularizing the different components
-# of the library so to make them easier to test and to maintain.
-#
-# How could them be used after the next major release:
-#
-#   >>> from openfisca_core import module
-#   >>> module.symbol()
-#
-# And for classes:
-#
-#   >>> from openfisca_core import module
-#   >>> module.Symbol()
-#
-# See: https://www.python.org/dev/peps/pep-0008/#imports
-
-from . import types
-from .entity import Entity
-from .group_entity import GroupEntity
-from .helpers import build_entity, find_role
-from .role import Role
-
-SingleEntity = Entity
-
-__all__ = [
-    "Entity",
-    "SingleEntity",
-    "GroupEntity",
-    "Role",
-    "build_entity",
-    "find_role",
-    "types",
-]
+"""Provides a way of representing the entities of a rule system.
+
+Each rule system is comprised by legislation and regulations to be applied upon
+"someone". In legal and economical terms, "someone" is referred to as people:
+individuals, families, tax households, companies, and so on.
+
+People can be either human or non-human, that is a legal entity, also referred
+to as a legal person. Human or non-human, a person is an atomic element of a
+rule system: for example, in most legislations, a salary is invariably owed
+to an indivual, and payroll taxes by a company, as a juridical person. In
+OpenFisca, that atomic element is represented as an :class:`.Entity`.
+
+In other cases, legal and regulatory rules are defined for groups or clusters
+of people: for example, income tax is usually due by a tax household, that is
+a group of individuals. There may also be fiduciary entities where the members,
+legal entities, are collectively liable for a property tax. In OpenFisca, those
+cluster elements are represented as a :class:`.GroupEntity`.
+
+In the latter case, the atomic members of a given group may have a different
+:class:`Role` in the context of a specific rule: for example, income tax
+is due, in some legislations, by a tax household, where we find different
+roles as the declarant, the spouse, the children, and so on…
+
+What's important is that each rule, or in OpenFisca, a :class:`.Variable`,
+is defined either for an :class:`.Entity` or for a :class:`.GroupEntity`,
+and in the latter case, the way the rule is going to be applied depends
+on the attributes and roles of the members of the group.
+
+Finally, there is a distiction to be made between the "abstract" entities
+described by in a rule system, for example an individual, as in "any"
+individual, and an actual individual, like Mauko, Andrea, Mehdi, Seiko,
+or José.
+
+This module provides tools for modelling the former. For the actual
+"simulation" or "application" of any given :class:`.Variable` to a
+concrete individual or group of individuals, see :class:`.Population`
+and :class:`.GroupPopulation`.
+
+Official Public API:
+    * :class:`.Entity`
+    * :class:`.GroupEntity`
+    * :class:`.Role`
+    * :func:`.build_entity`
+    * :func:`.check_role_validity`
+
+Deprecated:
+    * :meth:`.Entity.set_tax_benefit_system`
+    * :meth:`.Entity.get_variable`
+    * :meth:`.Entity.check_variable_defined_for_entity`
+    * :meth:`.Entity.check_role_validity`
+
+Note:
+    The ``deprecated`` features are kept so as to give time to users to
+    migrate, and could be definitely removed from the codebase in a future
+    major release.
+
+Note:
+    How imports are being used today::
+
+        from openfisca_core.entities import *  # Bad
+        from openfisca_core.entities.helpers import build_entity  # Bad
+        from openfisca_core.entities.role import Role  # Bad
+
+    The previous examples provoke cyclic dependency problems, that prevents us
+    from modularizing the different components of the library, so as to make
+    them easier to test and to maintain.
+
+    How could them be used after the next major release::
+
+        from openfisca_core import entities
+        from openfisca_core.entities import Role
+
+        Role()  # Good: import classes as publicly exposed
+        entities.build_entity()  # Good: use functions as publicly exposed
+
+    .. seealso:: `PEP8#Imports`_ and `OpenFisca's Styleguide`_.
+
+    .. _PEP8#Imports:
+        https://www.python.org/dev/peps/pep-0008/#imports
+
+    .. _OpenFisca's Styleguide:
+        https://github.com/openfisca/openfisca-core/blob/master/STYLEGUIDE.md
+
+"""
+
+# Official Public API
+
+from .entity import Entity  # noqa: F401
+from .group_entity import GroupEntity  # noqa: F401
+from .helpers import build_entity, check_role_validity  # noqa: F401
+from .role import Role  # noqa: F401
+
+__all__ = ["Entity", "GroupEntity", "Role"]
+__all__ = ["build_entity", "check_role_validity", *__all__]
diff --git a/openfisca_core/entities/_role_builder.py b/openfisca_core/entities/_role_builder.py
new file mode 100644
index 000000000..fbed5dbfd
--- /dev/null
+++ b/openfisca_core/entities/_role_builder.py
@@ -0,0 +1,105 @@
+from __future__ import annotations
+
+from typing import Iterable, Optional, Sequence, Type
+
+from openfisca_core.types import HasPlural, RoleLike, SupportsRole
+
+import dataclasses
+
+
+@dataclasses.dataclass(frozen=True)
+class RoleBuilder:
+    """Builds roles & sub-roles from a given input.
+
+    Attributes:
+        builder (:obj:`.Entity` or :obj:`.GroupEntity`):
+            A builder object.
+        buildee (:obj:`.Role`):
+            The objects to be built.
+
+    Args:
+        builder: A builder object.
+        buildee: The objects to be built.
+
+    Examples:
+        >>> from openfisca_core.entities import GroupEntity, Role
+
+        >>> group_entity = GroupEntity(
+        ...     "household",
+        ...     "households",
+        ...     "A household",
+        ...     "All the people who live together in the same place.",
+        ...     []
+        ...    )
+
+        >>> items = [{
+        ...     "key": "parent",
+        ...     "subroles": ["first_parent", "second_parent"],
+        ...     }]
+
+        >>> builder = RoleBuilder(group_entity, Role)
+
+        >>> repr(RoleBuilder)
+        "<class 'openfisca_core.entities._role_builder.RoleBuilder'>"
+
+        >>> repr(builder)
+        "<RoleBuilder(households, <class '...Role'>)>"
+
+        >>> str(builder)
+        "<RoleBuilder(households, <class '...Role'>)>"
+
+        >>> builder(items)
+        (<Role(parent)>,)
+
+    .. versionadded:: 35.7.0
+
+    """
+
+    __slots__ = ["builder", "buildee"]
+    builder: HasPlural
+    buildee: Type[SupportsRole]
+
+    def __repr__(self) -> str:
+        return f"<{self.__class__.__name__}({self.builder}, {self.buildee})>"
+
+    def __call__(self, items: Iterable[RoleLike]) -> Sequence[SupportsRole]:
+        """Builds a sub/role for each item in ``items``.
+
+        Args:
+            items: Role-like items, see :class:`.RoleLike`.
+
+        Returns:
+            A :obj:`list` of :obj:`.Role`.
+
+        .. versionadded:: 35.7.0
+
+        """
+
+        return tuple(self.build(item) for item in items)
+
+    def build(self, item: RoleLike) -> SupportsRole:
+        """Builds a role from ``item``.
+
+        Args:
+            item: A role-like item, see :class:`.RoleLike`.
+
+        Returns:
+            :obj:`.Role`: A :obj:`.Role`.
+
+        .. versionadded:: 35.7.0
+
+        """
+
+        role: SupportsRole
+        subroles: Optional[Iterable[str]]
+
+        role = self.buildee(item, self.builder)
+        subroles = item.get("subroles", [])
+
+        if subroles:
+            role.subroles = [
+                self.build(RoleLike({"key": key, "max": 1})) for key in subroles
+            ]
+            role.max = len(role.subroles)
+
+        return role
diff --git a/openfisca_core/entities/_variable_proxy.py b/openfisca_core/entities/_variable_proxy.py
new file mode 100644
index 000000000..7381ff90b
--- /dev/null
+++ b/openfisca_core/entities/_variable_proxy.py
@@ -0,0 +1,178 @@
+from __future__ import annotations
+
+from typing import Any, Optional, Type
+from typing_extensions import Protocol
+
+from openfisca_core.types import HasPlural, HasVariables, SupportsFormula
+
+import functools
+import os
+
+DOC_URL = "https://openfisca.org/doc/coding-the-legislation"
+
+E = HasPlural
+T = HasVariables
+V = SupportsFormula
+
+
+class Query(Protocol):
+    """A dummy class to "duck-check" :meth:`.TaxBenefitSystem.get_variable`."""
+
+    def __call__(
+        self,
+        __arg1: str,
+        __arg2: bool = False,
+    ) -> Optional["VariableProxy"]:
+        """See comment above."""
+
+        ...
+
+
+class VariableProxy:
+    """A `descriptor`_ to find an :obj:`.Entity`'s :obj:`.Variable`.
+
+    Attributes:
+        entity (:obj:`.Entity`, optional):
+            The :obj:`.Entity` ``owner`` of the descriptor.
+        tax_benefit_system (:obj:`.TaxBenefitSystem`, optional):
+            The :obj:`.Entity`'s :obj:`.TaxBenefitSystem`.
+        query (:meth:`.TaxBenefitSystem.get_variable`):
+            The method used to query the :obj:`.TaxBenefitSystem`.
+
+    Examples:
+        >>> from openfisca_core.entities import Entity
+        >>> from openfisca_core.taxbenefitsystems import TaxBenefitSystem
+        >>> from openfisca_core.variables import Variable
+
+        >>> entity = Entity(
+        ...     "individual",
+        ...     "individuals",
+        ...     "An individual",
+        ...     "The minimal legal entity on which a rule can be applied.",
+        ...    )
+
+        >>> class Variable(Variable):
+        ...     definition_period = "month"
+        ...     value_type = float
+        ...     entity = entity
+
+        >>> tbs = TaxBenefitSystem([entity])
+        >>> tbs.add_variable(Variable)
+        <openfisca_core.entities._variable_proxy.Variable...
+
+        >>> entity.tax_benefit_system = tbs
+
+        >>> entity.variables.get("Variable")
+        <...Variable...
+
+        >>> entity.variables.exists().get("Variable")
+        <...Variable...
+
+        >>> entity.variables.isdefined().get("Variable")
+        <...Variable...
+
+    .. _descriptor: https://docs.python.org/3/howto/descriptor.html
+
+    .. versionadded:: 35.7.0
+
+    """
+
+    entity: Optional[E] = None
+    tax_benefit_system: Optional[T] = None
+    query: Query
+
+    def __get__(self, entity: E, type: Type[E]) -> Optional[VariableProxy]:
+        """Binds :meth:`.TaxBenefitSystem.get_variable`."""
+
+        self.entity = entity
+
+        self.tax_benefit_system = getattr(
+            self.entity,
+            "tax_benefit_system",
+            None,
+        )
+
+        if self.tax_benefit_system is None:
+            return None
+
+        self.query = self.tax_benefit_system.get_variable
+
+        return self
+
+    def __set__(self, entity: E, value: Any) -> None:
+        NotImplemented
+
+    def get(self, variable_name: str) -> Optional[V]:
+        """Runs the query for ``variable_name``, based on the options given.
+
+        Args:
+            variable_name: The :obj:`.Variable` to be found.
+
+        Returns:
+            :obj:`.Variable` or :obj:`None`:
+            :obj:`.Variable` when the :obj:`.Variable` exists.
+            :obj:`None` when the :attr:`.tax_benefit_system` is not set.
+
+        Raises:
+            :exc:`.VariableNotFoundError`: When :obj:`.Variable` doesn't exist.
+            :exc:`.ValueError`: When the :obj:`.Variable` exists but is defined
+            for another :obj:`.Entity`.
+
+        .. versionadded:: 35.7.0
+
+        """
+
+        if self.entity is None:
+            return NotImplemented
+
+        return self.query(variable_name)
+
+    def exists(self) -> VariableProxy:
+        """Sets ``check_existence`` to ``True``."""
+
+        self.query = functools.partial(
+            self.query,
+            check_existence=True,
+        )
+
+        return self
+
+    def isdefined(self) -> VariableProxy:
+        """Checks that ``variable_name`` is defined for :attr:`.entity`."""
+
+        # We assume that we're also checking for existance.
+        self.exists()
+
+        self.query = functools.partial(
+            self._isdefined,
+            self.query,
+        )
+
+        return self
+
+    def _isdefined(self, query: Query, variable_name: str, **any: Any) -> Any:
+        variable = query(variable_name)
+
+        if self.entity is None:
+            return None
+
+        if variable is None:
+            return None
+
+        if variable.entity is None:
+            return None
+
+        if self.entity != variable.entity:
+            message = os.linesep.join(
+                [
+                    f"You tried to compute the variable '{variable_name}' for",
+                    f"the entity '{self.entity.plural}'; however the variable",
+                    f"'{variable_name}' is defined for the entity",
+                    f"'{variable.entity.plural}'. Learn more about entities",
+                    f"in our documentation: <{DOC_URL}/50_entities.html>.",
+                ]
+            )
+
+            raise ValueError(message)
+
+        return variable
diff --git a/openfisca_core/entities/entity.py b/openfisca_core/entities/entity.py
index c00191816..570bfdcca 100644
--- a/openfisca_core/entities/entity.py
+++ b/openfisca_core/entities/entity.py
@@ -1,42 +1,287 @@
-from typing import ClassVar
+from typing import Any, Iterator, Optional, Tuple
 
+from openfisca_core.types import Descriptor, HasHolders, HasVariables, SupportsFormula
+
+import dataclasses
 import textwrap
 
-from . import types as t
-from ._core_entity import _CoreEntity
+from openfisca_core.commons import deprecated
+
+from .. import entities
+from ._variable_proxy import VariableProxy
+
 
+@dataclasses.dataclass
+class Entity:
+    """Represents an entity on which calculations can be run.
 
-class Entity(_CoreEntity):
-    """An entity (e.g. a person, a household) on which calculations can be run.
+    For example an individual, a company, etc. An :class:`.Entity`
+    represents an "abstract" atomic unit of the legislation, as in
+    "any individual", or "any company".
+
+    Attributes:
+        key (:obj:`str`):
+            Key to identify the :class:`.Entity`.
+        plural (:obj:`str`):
+            The ``key``, pluralised.
+        label (:obj:`str`):
+            A summary description.
+        doc (:obj:`str`):
+            A full description, dedented.
+        is_person (:obj:`bool`):
+            Represents an individual? Defaults to True.
 
     Args:
-        key: A key to identify the ``Entity``.
-        plural: The ``key`` pluralised.
+        key: Key to identify the :class:`.Entity`.
+        plural: ``key``, pluralised.
         label: A summary description.
         doc: A full description.
 
-    """
+    Examples:
+        >>> entity = Entity(
+        ...     "individual",
+        ...     "individuals",
+        ...     "An individual",
+        ...     "The minimal legal entity on which a rule might be applied.",
+        ...    )
 
-    #: A key to identify the ``Entity``.
-    key: t.EntityKey
+        >>> repr(Entity)
+        "<class 'openfisca_core.entities.entity.Entity'>"
 
-    #: The ``key`` pluralised.
-    plural: t.EntityPlural
+        >>> repr(entity)
+        '<Entity(individual)>'
 
-    #: A summary description.
-    label: str
+        >>> str(entity)
+        'individuals'
+
+        >>> dict(entity)
+        {'key': 'individual', 'plural': 'individuals', 'label': 'An individ...}
+
+        >>> list(entity)
+        [('key', 'individual'), ('plural', 'individuals'), ('label', 'An in...]
+
+        >>> entity == entity
+        True
+
+        >>> entity != entity
+        False
+
+        :attr:`population`
+
+        >>> from openfisca_core.populations import Population
+
+        >>> entity.population = Population(entity)
+        >>> entity == entity.population.entity
+        True
+
+        :attr:`tax_benefit_system`
+
+        >>> from openfisca_core.taxbenefitsystems import TaxBenefitSystem
+
+        >>> entity.tax_benefit_system = TaxBenefitSystem([entity])
+        >>> entity in entity.tax_benefit_system.entities
+        True
+
+        :attr:`.variables`
 
-    #: A full description.
+        >>> from openfisca_core.variables import Variable
+
+        >>> class Variable(Variable):
+        ...     definition_period = "month"
+        ...     value_type = float
+        ...     entity = entity
+
+        >>> entity.variables.get("Variable")
+
+        >>> entity.tax_benefit_system.add_variable(Variable)
+        <...Variable...
+
+        >>> entity.variables.get("Variable")
+        <...Variable...
+
+        >>> entity.variables.exists().get("Variable")
+        <...Variable...
+
+        >>> entity.variables.isdefined().get("Variable")
+        <...Variable...
+
+    .. versionchanged:: 35.7.0
+        Hereafter :attr:`.variables` allows querying a :obj:`.TaxBenefitSystem`
+        for a :obj:`.Variable`.
+
+    .. versionchanged:: 35.7.0
+        Hereafter the equality of an :obj:`.Entity` is determined by its
+        data attributes.
+
+    """
+
+    __slots__ = [
+        "key",
+        "plural",
+        "label",
+        "doc",
+        "is_person",
+        "_population",
+        "_tax_benefit_system",
+    ]
+
+    key: str
+    plural: str
+    label: str
     doc: str
 
-    #: Whether the ``Entity`` is a person or not.
-    is_person: ClassVar[bool] = True
+    @property
+    def population(self) -> Optional[HasHolders]:
+        """An :class:`.Entity` has one :class:`.Population`."""
+
+        return self._population
+
+    @population.setter
+    def population(self, value: HasHolders) -> None:
+        self._population = value
+
+    @property
+    def tax_benefit_system(self) -> Optional[HasVariables]:
+        """An :class:`.Entity` belongs to a :obj:`.TaxBenefitSystem`."""
+
+        return self._tax_benefit_system
+
+    @tax_benefit_system.setter
+    def tax_benefit_system(self, value: HasVariables) -> None:
+        self._tax_benefit_system = value
+
+    @property
+    def variables(self) -> Optional[VariableProxy]:
+        """An :class:`.Entity` has many :class:`Variables <.Variable>`."""
+
+        return self._variables
+
+    _variables: Descriptor[VariableProxy] = dataclasses.field(
+        init=False,
+        compare=False,
+        default=VariableProxy(),
+    )
+
+    def __post_init__(self, *__: Any) -> None:
+        self.doc = textwrap.dedent(self.doc)
+        self.is_person = True
+
+    def __repr__(self) -> str:
+        return f"<{self.__class__.__name__}({self.key})>"
+
+    def __str__(self) -> str:
+        return self.plural
+
+    def __iter__(self) -> Iterator[Tuple[str, Any]]:
+        return (
+            (item, self.__getattribute__(item))
+            for item in self.__slots__
+            if not item.startswith("_")
+        )
+
+    @deprecated(since="35.7.0", expires="the future")
+    def set_tax_benefit_system(
+        self,
+        tax_benefit_system: HasVariables,
+    ) -> None:
+        """Sets ``_tax_benefit_system``.
+
+        Args:
+            tax_benefit_system: To query variables from.
+
+        .. deprecated:: 35.7.0
+            :meth:`.set_tax_benefit_system` has been deprecated and will be
+            removed in the future. The functionality is now provided by
+            :attr:`.tax_benefit_system`.
+
+        """
+
+        self.tax_benefit_system = tax_benefit_system
+
+    @deprecated(since="35.7.0", expires="the future")
+    def get_variable(
+        self,
+        variable_name: str,
+        check_existence: bool = False,
+    ) -> Optional[SupportsFormula]:
+        """Gets ``variable_name`` from ``variables``.
+
+        Args:
+            variable_name: The variable to be found.
+            check_existence: Was the variable found? Defaults to False.
+
+        Returns:
+            :obj:`.Variable` or :obj:`None`:
+            :obj:`.Variable` when the :obj:`.Variable` exists.
+            :obj:`None` when the :obj:`.Variable` doesn't exist.
+
+        Raises:
+            :exc:`.VariableNotFoundError`: When ``check_existence`` is True and
+            the :obj:`.Variable` doesn't exist.
+
+        .. deprecated:: 35.7.0
+            :meth:`.get_variable` has been deprecated and will be
+            removed in the future. The functionality is now provided by
+            ``variables``.
+
+        """
+
+        if self.variables is None:
+            return None
+
+        if check_existence:
+            return self.variables.exists().get(variable_name)
+
+        return self.variables.get(variable_name)
+
+    @deprecated(since="35.7.0", expires="the future")
+    def check_variable_defined_for_entity(
+        self,
+        variable_name: str,
+    ) -> Optional[SupportsFormula]:
+        """Checks if ``variable_name`` is defined for ``self``.
+
+        Args:
+            variable_name: The :obj:`.Variable` to be found.
+
+        Returns:
+            :obj:`.Variable` or :obj:`None`:
+            :obj:`.Variable` when the :obj:`.Variable` exists.
+            :obj:`None` when the :attr:`.tax_benefit_system` is not set.
+
+        Raises:
+            :exc:`.VariableNotFoundError`: When :obj:`.Variable` doesn't exist.
+            :exc:`.ValueError`: When the :obj:`.Variable` exists but is defined
+            for another :obj:`.Entity`.
+
+        .. deprecated:: 35.7.0
+            :meth:`.check_variable_defined_for_entity` has been deprecated and
+            will be removed in the future. The functionality is now provided by
+            ``variables``.
+
+        """
+
+        if self.variables is None:
+            return None
+
+        return self.variables.isdefined().get(variable_name)
+
+    @staticmethod
+    @deprecated(since="35.7.0", expires="the future")
+    def check_role_validity(role: Any) -> None:
+        """Checks if ``role`` is an instance of :class:`.Role`.
+
+        Args:
+            role: Any object.
+
+        Returns:
+            :obj:`None`.
 
-    def __init__(self, key: str, plural: str, label: str, doc: str) -> None:
-        self.key = t.EntityKey(key)
-        self.plural = t.EntityPlural(plural)
-        self.label = label
-        self.doc = textwrap.dedent(doc)
+        .. deprecated:: 35.7.0
+            :meth:`.check_role_validity` has been deprecated and will be
+            removed in the future. The functionality is now provided by
+            :func:`.entities.check_role_validity`.
 
+        """
 
-__all__ = ["Entity"]
+        return entities.check_role_validity(role)
diff --git a/openfisca_core/entities/group_entity.py b/openfisca_core/entities/group_entity.py
index 4b588567a..066273d87 100644
--- a/openfisca_core/entities/group_entity.py
+++ b/openfisca_core/entities/group_entity.py
@@ -1,81 +1,163 @@
-from __future__ import annotations
+from typing import Any, Dict, Iterable, Optional, Sequence
 
-from collections.abc import Iterable, Sequence
-from typing import ClassVar
+from openfisca_core.types import Builder, RoleLike, SupportsRole
 
+import dataclasses
 import textwrap
-from itertools import chain
 
-from . import types as t
-from ._core_entity import _CoreEntity
+from ._role_builder import RoleBuilder
+from .entity import Entity
 from .role import Role
 
 
-class GroupEntity(_CoreEntity):
-    """Represents an entity containing several others with different roles.
+@dataclasses.dataclass(repr=False)
+class GroupEntity(Entity):
+    """Represents a :class:`.GroupEntity` on which calculations can be run.
 
-    A ``GroupEntity`` represents an ``Entity`` containing several other entities,
-    with different roles, and on which calculations can be run.
+    A :class:`.GroupEntity` is basically a group of people, and thus it is
+    composed of several :obj:`Entity` with different :obj:`Role` within the
+    group. For example a tax household, a family, a trust, etc.
+
+    Attributes:
+        key (:obj:`str`): Key to identify the :class:`.GroupEntity`.
+        plural (:obj:`str`): The ``key``, pluralised.
+        label (:obj:`str`): A summary description.
+        doc (:obj:`str`): A full description, dedented.
+        is_person (:obj:`bool`): Represents an individual? Defaults to False.
 
     Args:
-        key: A key to identify the ``GroupEntity``.
-        plural: The ``key`` pluralised.
+        key: Key to identify the :class:`.GroupEntity`.
+        plural: ``key``, pluralised.
         label: A summary description.
         doc: A full description.
-        roles: The list of roles of the group entity.
-        containing_entities: The list of keys of group entities whose members
-            are guaranteed to be a superset of this group's entities.
+        roles: The list of :class:`.Role` of the :class:`.GroupEntity`.
+
+    Examples:
+        >>> roles = [{
+        ...     "key": "parent",
+        ...     "subroles": ["first_parent", "second_parent"],
+        ...     }]
+
+        >>> group_entity = GroupEntity(
+        ...     "household",
+        ...     "households",
+        ...     "A household",
+        ...     "All the people who live together in the same place.",
+        ...     roles,
+        ...    )
+
+        >>> repr(GroupEntity)
+        "<class 'openfisca_core.entities.group_entity.GroupEntity'>"
+
+        >>> repr(group_entity)
+        '<GroupEntity(household)>'
+
+        >>> str(group_entity)
+        'households'
+
+        >>> dict(group_entity)
+        {'key': 'household', 'plural': 'households', 'label': 'A household...}
+
+        >>> list(group_entity)
+        [('key', 'household'), ('plural', 'households'), ('label', 'A hous...]
+
+        >>> group_entity == group_entity
+        True
+
+        >>> group_entity != group_entity
+        False
+
+        :attr:`roles`
+
+        >>> group_entity.roles
+        (<Role(parent)>,)
+
+        >>> group_entity.PARENT
+        <Role(parent)>
+
+        :attr:`flattened_roles`
+
+        >>> group_entity.flattened_roles
+        (<Role(first_parent)>, <Role(second_parent)>)
+
+        >>> group_entity.FIRST_PARENT
+        <Role(first_parent)>
+
+    .. versionchanged:: 35.7.0
+        Hereafter :attr:`.variables` allows querying a :obj:`.TaxBenefitSystem`
+        for a :obj:`.Variable`.
+
+    .. versionchanged:: 35.7.0
+        Hereafter the equality of an :obj:`.GroupEntity` is determined by its
+        data attributes.
 
     """
 
-    #: A key to identify the ``Entity``.
-    key: t.EntityKey
-
-    #: The ``key`` pluralised.
-    plural: t.EntityPlural
-
-    #: A summary description.
-    label: str
-
-    #: A full description.
-    doc: str
-
-    #: The list of roles of the ``GroupEntity``.
-    roles: Iterable[Role]
-
-    #: Whether the entity is a person or not.
-    is_person: ClassVar[bool] = False
-
-    def __init__(
-        self,
-        key: str,
-        plural: str,
-        label: str,
-        doc: str,
-        roles: Sequence[t.RoleParams],
-        containing_entities: Iterable[str] = (),
-    ) -> None:
-        self.key = t.EntityKey(key)
-        self.plural = t.EntityPlural(plural)
-        self.label = label
-        self.doc = textwrap.dedent(doc)
-        self.roles_description = roles
-        self.roles: Iterable[Role] = ()
-        for role_description in roles:
-            role = Role(role_description, self)
-            setattr(self, role.key.upper(), role)
-            self.roles = (*self.roles, role)
-            if subroles := role_description.get("subroles"):
-                role.subroles = ()
-                for subrole_key in subroles:
-                    subrole = Role({"key": subrole_key, "max": 1}, self)
-                    setattr(self, subrole.key.upper(), subrole)
-                    role.subroles = (*role.subroles, subrole)
-                role.max = len(role.subroles)
-        self.flattened_roles = tuple(
-            chain.from_iterable(role.subroles or [role] for role in self.roles),
+    __slots__ = [
+        "key",
+        "plural",
+        "label",
+        "doc",
+        "is_person",
+        "roles_description",
+        "_population",
+        "_tax_benefit_system",
+        "_roles",
+        "_roles_map",
+        "_flattened_roles",
+    ]
+
+    __: dataclasses.InitVar[Iterable[RoleLike]]
+
+    @property
+    def roles(self) -> Sequence[SupportsRole]:
+        """List of the roles of the group entity."""
+
+        return self._roles
+
+    @roles.setter
+    def roles(self, roles: Sequence[SupportsRole]) -> None:
+        self._roles = roles
+
+    @property
+    def flattened_roles(self) -> Sequence[SupportsRole]:
+        """:attr:`roles` flattened out."""
+
+        return self._flattened_roles
+
+    @flattened_roles.setter
+    def flattened_roles(self, roles: Sequence[SupportsRole]) -> None:
+        self._flattened_roles = tuple(
+            array for role in roles for array in role.subroles or [role]
         )
-        self.containing_entities = containing_entities
 
+    def __post_init__(self, *__: Iterable[RoleLike]) -> None:
+        self.doc = textwrap.dedent(self.doc)
+        self.is_person = False
+
+        # Extract roles.
+        roles: Iterable[RoleLike]
+        roles = next(iter(__))
+
+        # Build roles & sub-roles.
+        builder: Builder[GroupEntity, SupportsRole, RoleLike]
+        builder = RoleBuilder(self, Role)
+        self.roles = builder(roles)
+        self.flattened_roles = self.roles
+
+        # Assign role & sub-role attributes.
+        self._roles_map: Dict[str, SupportsRole]
+        self._roles_map = {}
+
+        for role in (*self.roles, *self.flattened_roles):
+            self._roles_map.update({role.key.upper(): role})
+
+        # Useless step kept to avoid changing the signature.
+        self.roles_description: Optional[Iterable[RoleLike]]
+        self.roles_description = roles
+
+    def __getattr__(self, attr: str) -> Any:
+        if attr.isupper():
+            return self._roles_map[attr]
 
-__all__ = ["GroupEntity"]
+        return self.__getattribute__(attr)
diff --git a/openfisca_core/entities/helpers.py b/openfisca_core/entities/helpers.py
index 146ab6d25..cf566f3df 100644
--- a/openfisca_core/entities/helpers.py
+++ b/openfisca_core/entities/helpers.py
@@ -1,9 +1,8 @@
-from __future__ import annotations
+from typing import Any, Iterable, Optional
 
-from collections.abc import Iterable, Sequence
+from openfisca_core.types import HasPlural, RoleLike, SupportsRole
 
-from . import types as t
-from .entity import Entity as SingleEntity
+from .entity import Entity
 from .group_entity import GroupEntity
 
 
@@ -12,154 +11,78 @@ def build_entity(
     plural: str,
     label: str,
     doc: str = "",
-    roles: None | Sequence[t.RoleParams] = None,
+    roles: Optional[Iterable[RoleLike]] = None,
     is_person: bool = False,
-    *,
-    class_override: object = None,
-    containing_entities: Sequence[str] = (),
-) -> t.SingleEntity | t.GroupEntity:
-    """Build an ``Entity`` or a ``GroupEntity``.
+    class_override: Optional[Any] = None,
+) -> HasPlural:
+    """Builds an :class:`.Entity` or a :class:`.GroupEntity`.
 
     Args:
-        key: Key to identify the ``Entity`` or ``GroupEntity``.
-        plural: The ``key`` pluralised.
+        key: Key to identify the :class:`.Entity` or :class:`.GroupEntity`.
+        plural: ``key``, pluralised.
         label: A summary description.
         doc: A full description.
-        roles: A list of roles —if it's a ``GroupEntity``.
+        roles: A list of :class:`.Role`, if it's a :class:`.GroupEntity`.
         is_person: If is an individual, or not.
         class_override: ?
-        containing_entities: Keys of contained entities.
 
     Returns:
-        Entity: When ``is_person`` is ``True``.
-        GroupEntity: When ``is_person`` is ``False``.
+        :obj:`.Entity` or :obj:`.GroupEntity`:
+        :obj:`.Entity`: When ``is_person`` is True.
+        :obj:`.GroupEntity`: When ``is_person`` is False.
 
     Raises:
-        NotImplementedError: If ``roles`` is ``None``.
+        ValueError: If ``roles`` is not iterable.
 
     Examples:
-        >>> from openfisca_core import entities
-
-        >>> entity = build_entity(
+        >>> build_entity(
         ...     "syndicate",
         ...     "syndicates",
         ...     "Banks loaning jointly.",
-        ...     roles=[],
-        ...     containing_entities=(),
-        ... )
-        >>> entity
-        GroupEntity(syndicate)
+        ...     roles = [],
+        ...     )
+        <GroupEntity(syndicate)>
 
         >>> build_entity(
         ...     "company",
         ...     "companies",
         ...     "A small or medium company.",
-        ...     is_person=True,
-        ... )
-        Entity(company)
+        ...     is_person = True,
+        ...     )
+        <Entity(company)>
 
-        >>> role = entities.Role({"key": "key"}, entity)
-
-        >>> build_entity(
-        ...     "syndicate",
-        ...     "syndicates",
-        ...     "Banks loaning jointly.",
-        ...     roles=[role],
-        ... )
-        Traceback (most recent call last):
-        TypeError: 'Role' object is not subscriptable
+    .. versionchanged:: 35.7.0
+        Instead of raising :exc:`TypeError` when ``roles`` is None, it does
+        now raise :exc:`ValueError` when ``roles`` is not iterable.
 
     """
 
     if is_person:
-        return SingleEntity(key, plural, label, doc)
+        return Entity(key, plural, label, doc)
 
-    if roles is not None:
-        return GroupEntity(
-            key,
-            plural,
-            label,
-            doc,
-            roles,
-            containing_entities=containing_entities,
-        )
+    if roles is not None and hasattr(roles, "__iter__"):
+        return GroupEntity(key, plural, label, doc, roles)
 
-    raise NotImplementedError
+    raise ValueError(f"Invalid value '{roles}' for 'roles', must be iterable.")
 
 
-def find_role(
-    roles: Iterable[t.Role],
-    key: t.RoleKey,
-    *,
-    total: None | int = None,
-) -> None | t.Role:
-    """Find a ``Role`` in a ``GroupEntity``.
+def check_role_validity(role: Any) -> None:
+    """Checks if ``role`` is an instance of :class:`.Role`.
 
     Args:
-        roles: The roles to search.
-        key: The key of the role to find.
-        total: The ``max`` attribute of the role to find.
+        role: Any object.
 
-    Returns:
-        Role: The role if found
-        None: Else ``None``.
+    Raises:
+        ValueError: When ``role`` is not a :class:`.Role`.
 
     Examples:
-        >>> from openfisca_core.entities.types import RoleParams
-
-        >>> principal = RoleParams(
-        ...     key="principal",
-        ...     label="Principal",
-        ...     doc="Person focus of a calculation in a family context.",
-        ...     max=1,
-        ... )
-
-        >>> partner = RoleParams(
-        ...     key="partner",
-        ...     plural="partners",
-        ...     label="Partners",
-        ...     doc="Persons partners of the principal.",
-        ... )
-
-        >>> parent = RoleParams(
-        ...     key="parent",
-        ...     plural="parents",
-        ...     label="Parents",
-        ...     doc="Persons parents of children of the principal",
-        ...     subroles=["first_parent", "second_parent"],
-        ... )
-
-        >>> group_entity = build_entity(
-        ...     key="family",
-        ...     plural="families",
-        ...     label="Family",
-        ...     doc="A Family represents a collection of related persons.",
-        ...     roles=[principal, partner, parent],
-        ... )
-
-        >>> find_role(group_entity.roles, "principal", total=1)
-        Role(principal)
-
-        >>> find_role(group_entity.roles, "partner")
-        Role(partner)
-
-        >>> find_role(group_entity.roles, "parent", total=2)
-        Role(parent)
-
-        >>> find_role(group_entity.roles, "first_parent", total=1)
-        Role(first_parent)
-
-    """
-    for role in roles:
-        if role.subroles:
-            for subrole in role.subroles:
-                if (subrole.max == total) and (subrole.key == key):
-                    return subrole
-
-        if (role.max == total) and (role.key == key):
-            return role
+        >>> from openfisca_core.entities import Role
+        >>> role = Role({"key": "key"}, object())
+        >>> check_role_validity(role)
 
-    return None
+    .. versionadded:: 35.7.0
 
+    """
 
-__all__ = ["build_entity", "find_role"]
+    if role is not None and not isinstance(role, SupportsRole):
+        raise ValueError(f"{role} is not a valid role")
diff --git a/openfisca_core/entities/role.py b/openfisca_core/entities/role.py
index e687b2604..5fdea85bb 100644
--- a/openfisca_core/entities/role.py
+++ b/openfisca_core/entities/role.py
@@ -1,91 +1,92 @@
 from __future__ import annotations
 
-from collections.abc import Iterable
+from typing import Any, Iterator, Optional, Sequence, Tuple
 
-from . import types as t
-from ._description import _Description
+from openfisca_core.types import HasPlural, RoleLike, SupportsRole
 
+import dataclasses
+import textwrap
 
-class Role:
-    """The role of an ``Entity`` within a ``GroupEntity``.
 
-    Each ``Entity`` related to a ``GroupEntity`` has a ``Role``. For example,
-    if you have a family, its roles could include a parent, a child, and so on.
-    Or if you have a tax household, its roles could include the taxpayer, a
-    spouse, several dependents, and the like.
+@dataclasses.dataclass(init=False)
+class Role:
+    """Role of an :class:`.Entity` within a :class:`.GroupEntity`.
+
+    Each :class:`.Entity` related to a :class:`.GroupEntity` has a
+    :class:`.Role`. For example, if you have a family, its roles could include
+    a parent, a child, and so on. Or if you have a tax household, its roles
+    could include the taxpayer, a spouse, several dependents, and so on.
+
+    Attributes:
+        entity (:obj:`.GroupEntity`): Entity the :class:`.Role` belongs to.
+        key (:obj:`str`): Key to identify the :class:`.Role`.
+        plural (:obj:`str`, optional): The ``key``, pluralised.
+        label (:obj:`str`, optional): A summary description.
+        doc (:obj:`str`): A full description, dedented.
+        max (:obj:`int`, optional): Max number of members. Defaults to None.
+        subroles (list, optional): The ``subroles``. Defaults to None.
 
     Args:
-        description: A description of the Role.
-        entity: The Entity to which the Role belongs.
+        description: A dictionary containing most of the attributes.
+        entity: :obj:`.Entity` the :class:`.Role` belongs to.
 
     Examples:
-        >>> from openfisca_core import entities
-        >>> entity = entities.GroupEntity("key", "plural", "label", "doc", [])
-        >>> role = entities.Role({"key": "parent"}, entity)
+        >>> description = {
+        ...     "key": "parent",
+        ...     "label": "Parents",
+        ...     "plural": "parents",
+        ...     "doc": "The one or two adults in charge of the household.",
+        ...     "max": 2,
+        ...     }
+
+        >>> role = Role(description, object())
 
         >>> repr(Role)
         "<class 'openfisca_core.entities.role.Role'>"
 
         >>> repr(role)
-        'Role(parent)'
+        '<Role(parent)>'
 
         >>> str(role)
-        'Role(parent)'
+        'parent'
 
-        >>> {role}
-        {Role(parent)}
+        >>> dict(role)
+        {'entity': <object object...>, 'key': 'parent', 'plural': 'parents'...}
 
-        >>> role.key
-        'parent'
+        >>> list(role)
+        [('entity', <object object...>), ('key', 'parent'), ('plural', 'par...]
+
+        >>> role == role
+        True
+
+        >>> role != role
+        False
 
     """
 
-    #: The ``GroupEntity`` the Role belongs to.
-    entity: t.GroupEntity
-
-    #: A description of the ``Role``.
-    description: _Description
-
-    #: Max number of members.
-    max: None | int = None
-
-    #: A list of subroles.
-    subroles: None | Iterable[Role] = None
-
-    @property
-    def key(self) -> t.RoleKey:
-        """A key to identify the ``Role``."""
-        return t.RoleKey(self.description.key)
-
-    @property
-    def plural(self) -> None | t.RolePlural:
-        """The ``key`` pluralised."""
-        if (plural := self.description.plural) is None:
-            return None
-        return t.RolePlural(plural)
-
-    @property
-    def label(self) -> None | str:
-        """A summary description."""
-        return self.description.label
-
-    @property
-    def doc(self) -> None | str:
-        """A full description, non-indented."""
-        return self.description.doc
-
-    def __init__(self, description: t.RoleParams, entity: t.GroupEntity) -> None:
-        self.description = _Description(
-            key=description["key"],
-            plural=description.get("plural"),
-            label=description.get("label"),
-            doc=description.get("doc"),
-        )
+    __slots__ = ["entity", "key", "plural", "label", "doc", "max", "subroles"]
+    entity: HasPlural
+    key: str
+    plural: Optional[str]
+    label: Optional[str]
+    doc: str
+    max: Optional[int]
+    subroles: Optional[Sequence[SupportsRole]]
+
+    def __init__(self, description: RoleLike, entity: HasPlural) -> None:
         self.entity = entity
+        self.key = description["key"]
+        self.plural = description.get("plural")
+        self.label = description.get("label")
+        self.doc = textwrap.dedent(str(description.get("doc", "")))
         self.max = description.get("max")
+        self.subroles = None
 
     def __repr__(self) -> str:
-        return f"Role({self.key})"
+        return f"<{self.__class__.__name__}({self.key})>"
 
+    def __str__(self) -> str:
+        return self.key
 
-__all__ = ["Role"]
+    def __iter__(self) -> Iterator[Tuple[str, Any]]:
+        return ((item, self.__getattribute__(item)) for item in self.__slots__)
diff --git a/openfisca_core/entities/tests/test_entity.py b/openfisca_core/entities/tests/test_entity.py
index b3cb813dd..dc2154287 100644
--- a/openfisca_core/entities/tests/test_entity.py
+++ b/openfisca_core/entities/tests/test_entity.py
@@ -1,10 +1,56 @@
-from openfisca_core import entities
+import pytest
 
+from openfisca_core.entities import Entity, Role
+from openfisca_core.taxbenefitsystems import TaxBenefitSystem
+
+
+@pytest.fixture
+def entity():
+    """An entity."""
+
+    return Entity("key", "label", "plural", "doc")
+
+
+@pytest.fixture
+def role(entity):
+    """A role."""
+
+    return Role({"key": "key"}, entity)
+
+
+def test_init_when_doc_indented():
+    """Dedents the ``doc`` attribute if it is passed at initialisation."""
 
-def test_init_when_doc_indented() -> None:
-    """De-indent the ``doc`` attribute if it is passed at initialisation."""
     key = "\tkey"
     doc = "\tdoc"
-    entity = entities.Entity(key, "label", "plural", doc)
+    entity = Entity(key, "label", "plural", doc)
     assert entity.key == key
-    assert entity.doc == doc.lstrip()
+    assert entity.doc != doc
+
+
+def test_set_tax_benefit_system_deprecation(entity):
+    """Throws a deprecation warning when called."""
+
+    with pytest.warns(DeprecationWarning):
+        entity.set_tax_benefit_system(TaxBenefitSystem([entity]))
+
+
+def test_check_role_validity_deprecation(entity, role):
+    """Throws a deprecation warning when called."""
+
+    with pytest.warns(DeprecationWarning):
+        entity.check_role_validity(role)
+
+
+def test_check_variable_defined_for_entity_deprecation(entity):
+    """Throws a deprecation warning when called."""
+
+    with pytest.warns(DeprecationWarning):
+        entity.check_variable_defined_for_entity(object())
+
+
+def test_get_variable_deprecation(entity):
+    """Throws a deprecation warning when called."""
+
+    with pytest.warns(DeprecationWarning):
+        entity.get_variable("variable")
diff --git a/openfisca_core/entities/tests/test_group_entity.py b/openfisca_core/entities/tests/test_group_entity.py
index 092c9d357..e27b07847 100644
--- a/openfisca_core/entities/tests/test_group_entity.py
+++ b/openfisca_core/entities/tests/test_group_entity.py
@@ -1,70 +1,65 @@
-from collections.abc import Mapping
-from typing import Any
-
 import pytest
 
-from openfisca_core import entities
+from openfisca_core.entities import Entity, GroupEntity
+from openfisca_core.taxbenefitsystems import TaxBenefitSystem
 
 
 @pytest.fixture
-def parent() -> str:
-    return "parent"
+def roles():
+    """A role-like."""
+
+    return [{"key": "parent", "subroles": ["first_parent", "second_parent"]}]
 
 
 @pytest.fixture
-def uncle() -> str:
-    return "uncle"
+def entity():
+    """An entity."""
+
+    return Entity("key", "label", "plural", "doc")
 
 
 @pytest.fixture
-def first_parent() -> str:
-    return "first_parent"
+def group_entity(roles):
+    """A group entity."""
 
+    return GroupEntity("key", "label", "plural", "doc", roles)
 
-@pytest.fixture
-def second_parent() -> str:
-    return "second_parent"
 
+def test_init_when_doc_indented():
+    """Dedents the ``doc`` attribute if it is passed at initialisation."""
 
-@pytest.fixture
-def third_parent() -> str:
-    return "third_parent"
+    key = "\tkey"
+    doc = "\tdoc"
+    group_entity = GroupEntity(key, "label", "plural", doc, [])
+    assert group_entity.key == key
+    assert group_entity.doc != doc
 
 
-@pytest.fixture
-def role(parent: str, first_parent: str, third_parent: str) -> Mapping[str, Any]:
-    return {"key": parent, "subroles": {first_parent, third_parent}}
+def test_set_tax_benefit_system_deprecation(entity, group_entity):
+    """Throws a deprecation warning when called."""
 
+    tbs = TaxBenefitSystem([entity, group_entity])
 
-@pytest.fixture
-def group_entity(role: Mapping[str, Any]) -> entities.GroupEntity:
-    return entities.GroupEntity("key", "label", "plural", "doc", (role,))
+    with pytest.warns(DeprecationWarning):
+        group_entity.set_tax_benefit_system(tbs)
 
 
-def test_init_when_doc_indented() -> None:
-    """De-indent the ``doc`` attribute if it is passed at initialisation."""
-    key = "\tkey"
-    doc = "\tdoc"
-    group_entity = entities.GroupEntity(key, "label", "plural", doc, ())
-    assert group_entity.key == key
-    assert group_entity.doc == doc.lstrip()
-
-
-def test_group_entity_with_roles(
-    group_entity: entities.GroupEntity,
-    parent: str,
-    uncle: str,
-) -> None:
-    """Assign a Role for each role-like passed as argument."""
-    assert hasattr(group_entity, parent.upper())
-    assert not hasattr(group_entity, uncle.upper())
-
-
-def test_group_entity_with_subroles(
-    group_entity: entities.GroupEntity,
-    first_parent: str,
-    second_parent: str,
-) -> None:
-    """Assign a Role for each subrole-like passed as argument."""
-    assert hasattr(group_entity, first_parent.upper())
-    assert not hasattr(group_entity, second_parent.upper())
+def test_check_role_validity_deprecation(group_entity):
+    """Throws a deprecation warning when called."""
+
+    with pytest.warns(DeprecationWarning):
+        group_entity.check_role_validity(group_entity.PARENT)
+
+
+def test_check_variable_defined_for_entity_deprecation(group_entity):
+    """Throws a deprecation warning when called."""
+
+    with pytest.warns(DeprecationWarning):
+        group_entity.check_variable_defined_for_entity(object())
+
+
+def test_get_variable_deprecation(group_entity):
+    """Throws a deprecation warning when called."""
+
+    with pytest.warns(DeprecationWarning):
+        group_entity.get_variable("variable")
diff --git a/openfisca_core/entities/tests/test_helpers.py b/openfisca_core/entities/tests/test_helpers.py
new file mode 100644
index 000000000..c6eedb122
--- /dev/null
+++ b/openfisca_core/entities/tests/test_helpers.py
@@ -0,0 +1,17 @@
+import pytest
+
+from openfisca_core import entities
+
+
+def test_build_entity_without_roles():
+    """Raises a ArgumentError when it's called without roles."""
+
+    with pytest.raises(ValueError):
+        entities.build_entity("", "", "", roles=None)
+
+
+def test_check_role_validity_when_not_role():
+    """Raises a ValueError when it gets an invalid role."""
+
+    with pytest.raises(ValueError):
+        entities.check_role_validity(object())
diff --git a/openfisca_core/entities/tests/test_role.py b/openfisca_core/entities/tests/test_role.py
index 454d862c7..0953a8689 100644
--- a/openfisca_core/entities/tests/test_role.py
+++ b/openfisca_core/entities/tests/test_role.py
@@ -1,11 +1,11 @@
-from openfisca_core import entities
+from openfisca_core.entities import Role
 
 
-def test_init_when_doc_indented() -> None:
-    """De-indent the ``doc`` attribute if it is passed at initialisation."""
+def test_init_when_doc_indented():
+    """Dedents the ``doc`` attribute if it is passed at initialisation."""
+
     key = "\tkey"
     doc = "\tdoc"
-    entity = entities.GroupEntity("key", "plural", "label", "doc", [])
-    role = entities.Role({"key": key, "doc": doc}, entity)
+    role = Role({"key": key, "doc": doc}, object())
     assert role.key == key
-    assert role.doc == doc.lstrip()
+    assert role.doc != doc
diff --git a/openfisca_core/entities/tests/test_variable_proxy.py b/openfisca_core/entities/tests/test_variable_proxy.py
new file mode 100644
index 000000000..f9f571819
--- /dev/null
+++ b/openfisca_core/entities/tests/test_variable_proxy.py
@@ -0,0 +1,159 @@
+import pytest
+
+from openfisca_core.entities import Entity
+from openfisca_core.errors import VariableNotFoundError
+from openfisca_core.taxbenefitsystems import TaxBenefitSystem
+from openfisca_core.variables import Variable
+
+from .._variable_proxy import VariableProxy
+
+
+@pytest.fixture()
+def variables():
+    """A variable descriptor."""
+
+    return VariableProxy()
+
+
+@pytest.fixture
+def entity():
+    """An entity."""
+
+    return Entity("", "individuals", "", "")
+
+
+@pytest.fixture
+def ThisVar(entity):
+    """A variable."""
+
+    return type(
+        "ThisVar",
+        (Variable,),
+        {
+            "definition_period": "month",
+            "value_type": float,
+            "entity": entity,
+        },
+    )
+
+
+@pytest.fixture
+def ThatVar():
+    """Another variable."""
+
+    class ThatVar(Variable):
+        definition_period = "month"
+        value_type = float
+        entity = Entity("", "martians", "", "")
+
+    return ThatVar
+
+
+@pytest.fixture
+def tbs(entity, ThisVar, ThatVar):
+    """A tax-benefit system."""
+
+    tbs = TaxBenefitSystem([entity])
+    tbs.add_variable(ThisVar)
+    tbs.add_variable(ThatVar)
+    return tbs
+
+
+def test_variables_without_variable_name(variables):
+    """Raises a TypeError when called without ``variable_name``."""
+
+    with pytest.raises(TypeError, match="'variable_name'"):
+        variables.get()
+
+
+def test_variables_without_owner(variables):
+    """Returns NotImplemented when called without an ``owner``."""
+
+    assert variables.get("ThisVar") == NotImplemented
+
+
+def test_variables_setter(entity):
+    """Raises AttributeError when tryng to set ``variables``."""
+
+    with pytest.raises(AttributeError, match="can't set attribute"):
+        entity.variables = object()
+
+
+def test_variables_without_tax_benefit_system(entity):
+    """Returns None when called without a TaxBenefitSystem."""
+
+    assert not entity.variables
+
+
+def test_variables_when_exists(entity, tbs, ThisVar):
+    """Returns the variable when it exists."""
+
+    entity.tax_benefit_system = tbs
+    variable = entity.variables.get("ThisVar")
+    assert isinstance(variable, ThisVar)
+
+
+def test_variables_when_doesnt_exist(entity, tbs):
+    """Returns None when it does not."""
+
+    entity.tax_benefit_system = tbs
+    assert not entity.variables.get("OtherVar")
+
+
+def test_variables_when_exists_and_check_exists(entity, tbs, ThisVar):
+    """Raises a VariableNotFoundError when checking for existance."""
+
+    entity.tax_benefit_system = tbs
+    variable = entity.variables.exists().get("ThisVar")
+    assert isinstance(variable, ThisVar)
+
+
+def test_variables_when_doesnt_exist_and_check_exists(entity, tbs):
+    """Raises a VariableNotFoundError when checking for existance."""
+
+    entity.tax_benefit_system = tbs
+
+    with pytest.raises(VariableNotFoundError, match="'OtherVar'"):
+        entity.variables.exists().get("OtherVar")
+
+
+def test_variables_when_exists_and_defined_for(entity, tbs, ThisVar):
+    """Returns the variable when it exists and defined for the entity."""
+
+    entity.tax_benefit_system = tbs
+    variable = entity.variables.isdefined().get("ThisVar")
+    assert isinstance(variable, ThisVar)
+
+
+def test_variables_when_exists_and_not_defined_for(entity, tbs):
+    """Raises a ValueError when it exists but defined for another var."""
+
+    entity.tax_benefit_system = tbs
+
+    with pytest.raises(ValueError, match="'martians'"):
+        entity.variables.isdefined().get("ThatVar")
+
+
+def test_variables_when_doesnt_exist_and_check_defined_for(entity, tbs):
+    """Raises a VariableNotFoundError when it doesn't exist."""
+
+    entity.tax_benefit_system = tbs
+
+    with pytest.raises(VariableNotFoundError, match="'OtherVar'"):
+        entity.variables.isdefined().get("OtherVar")
+
+
+def test_variables_composition(entity, tbs):
+    """Conditions can be composed."""
+
+    entity.tax_benefit_system = tbs
+
+    assert entity.variables.exists().isdefined().get("ThisVar")
+
+
+def test_variables_permutation(entity, tbs):
+    """Conditions can be permuted."""
+
+    entity.tax_benefit_system = tbs
+
+    assert entity.variables.isdefined().exists().get("ThisVar")
diff --git a/openfisca_core/indexed_enums/__init__.py b/openfisca_core/indexed_enums/__init__.py
index a6a452511..5bc869f0c 100644
--- a/openfisca_core/indexed_enums/__init__.py
+++ b/openfisca_core/indexed_enums/__init__.py
@@ -1,13 +1,8 @@
 """Enumerations for variables with a limited set of possible values."""
 
-from . import types
 from .config import ENUM_ARRAY_DTYPE
 from .enum import Enum
 from .enum_array import EnumArray
+from . import types
 
-__all__ = [
-    "ENUM_ARRAY_DTYPE",
-    "Enum",
-    "EnumArray",
-    "types",
-]
+__all__ = ["ENUM_ARRAY_DTYPE", "Enum", "EnumArray", "types"]
diff --git a/openfisca_core/indexed_enums/enum.py b/openfisca_core/indexed_enums/enum.py
index 56f007941..e20209558 100644
--- a/openfisca_core/indexed_enums/enum.py
+++ b/openfisca_core/indexed_enums/enum.py
@@ -1,10 +1,11 @@
 from __future__ import annotations
 
+
 import numpy
 
-from . import types as t
-from .config import ENUM_ARRAY_DTYPE
 from .enum_array import EnumArray
+from .config import ENUM_ARRAY_DTYPE
+from . import types as t
 
 
 class Enum(t.Enum):
@@ -13,6 +14,67 @@ class Enum(t.Enum):
     Its items have an :class:`int` index, useful and performant when running
     :mod:`~openfisca_core.simulations` on large :mod:`~openfisca_core.populations`.
 
+    Examples:
+        >>> class Housing(Enum):
+        ...     OWNER = "Owner"
+        ...     TENANT = "Tenant"
+        ...     FREE_LODGER = "Free lodger"
+        ...     HOMELESS = "Homeless"
+
+        >>> repr(Housing)
+        "<enum 'Housing'>"
+
+        >>> repr(Housing.TENANT)
+        '<Housing.TENANT(Tenant)>'
+
+        >>> str(Housing.TENANT)
+        'Housing.TENANT'
+
+        >>> dict([(Housing.TENANT, Housing.TENANT.value)])
+        {<Housing.TENANT(Tenant)>: 'Tenant'}
+
+        >>> list(Housing)
+        [<Housing.OWNER(Owner)>, <Housing.TENANT(Tenant)>, ...]
+
+        >>> Housing["TENANT"]
+        <Housing.TENANT(Tenant)>
+
+        >>> Housing("Tenant")
+        <Housing.TENANT(Tenant)>
+
+        >>> Housing.TENANT in Housing
+        True
+
+        >>> len(Housing)
+        4
+
+        >>> Housing.TENANT == Housing.TENANT
+        True
+
+        >>> Housing.TENANT != Housing.TENANT
+        False
+
+        >>> Housing.TENANT > Housing.TENANT
+        False
+
+        >>> Housing.TENANT < Housing.TENANT
+        False
+
+        >>> Housing.TENANT >= Housing.TENANT
+        True
+
+        >>> Housing.TENANT <= Housing.TENANT
+        True
+
+        >>> Housing.TENANT.index
+        1
+
+        >>> Housing.TENANT.name
+        'TENANT'
+
+        >>> Housing.TENANT.value
+        'Tenant'
+
     """
 
     #: The :attr:`index` of the :class:`.Enum` member.
@@ -31,11 +93,60 @@ def __init__(self, *__args: object, **__kwargs: object) -> None:
         Note:
             ``_member_names_`` is undocumented in upstream :class:`enum.Enum`.
 
+        Examples:
+            >>> MyEnum = Enum("MyEnum", "foo bar")
+            >>> MyEnum.bar.index
+            1
+
+            >>> class MyEnum(Enum):
+            ...     foo = b"foo"
+            ...     bar = b"bar"
+
+            >>> MyEnum.bar.index
+            1
+
+            >>> array = numpy.array([[1, 2], [3, 4]])
+            >>> array[MyEnum.bar.index]
+            array([3, 4])
+
         """
         self.index = len(self._member_names_)
 
-    #: Bypass the slow :meth:`enum.Enum.__eq__` method.
-    __eq__ = object.__eq__
+    def __repr__(self) -> str:
+        return f"{self.__class__.__name__}.{self.name}({self.value})"
+
+    def __str__(self) -> str:
+        return f"{self.__class__.__name__}.{self.name}"
+
+    def __eq__(self, other: object) -> bool:
+        if not isinstance(other, Enum):
+            return NotImplemented
+        return self.index == other.index
+
+    def __ne__(self, other: object) -> bool:
+        if not isinstance(other, Enum):
+            return NotImplemented
+        return self.index != other.index
+
+    def __lt__(self, other: object) -> bool:
+        if not isinstance(other, Enum):
+            return NotImplemented
+        return self.index < other.index
+
+    def __le__(self, other: object) -> bool:
+        if not isinstance(other, Enum):
+            return NotImplemented
+        return self.index <= other.index
+
+    def __gt__(self, other: object) -> bool:
+        if not isinstance(other, Enum):
+            return NotImplemented
+        return self.index > other.index
+
+    def __ge__(self, other: object) -> bool:
+        if not isinstance(other, Enum):
+            return NotImplemented
+        return self.index >= other.index
 
     #: :meth:`.__hash__` must also be defined so as to stay hashable.
     __hash__ = object.__hash__
@@ -53,50 +164,94 @@ def encode(
         Returns:
             EnumArray: An :class:`.EnumArray` with the encoded input values.
 
-        For instance:
+        Examples:
+            >>> class MyEnum(Enum):
+            ...     foo = b"foo"
+            ...     bar = b"bar"
+
+            # EnumArray
+
+            >>> array = numpy.array([1])
+            >>> enum_array = EnumArray(array, MyEnum)
+            >>> MyEnum.encode(enum_array)
+            <EnumArray([<MyEnum.bar(b'bar')>])>
+
+            # ArrayTipe[Enum]
+
+            >>> array = numpy.array([MyEnum.bar])
+            >>> enum_array = MyEnum.encode(array)
+            >>> enum_array[0] == MyEnum.bar.index
+            True
+
+            # ArrayType[bytes]
+
+            >>> array = numpy.array([b"bar"])
+            >>> enum_array = MyEnum.encode(array)
+            >>> enum_array[0] == MyEnum.bar.index
+            True
+
+            # ArrayType[int]
 
-        >>> string_identifier_array = asarray(["free_lodger", "owner"])
-        >>> encoded_array = HousingOccupancyStatus.encode(string_identifier_array)
-        >>> encoded_array[0]
-        2  # Encoded value
+            >>> array = numpy.array([1])
+            >>> enum_array = MyEnum.encode(array)
+            >>> enum_array[0] == MyEnum.bar.index
+            True
+
+            # ArrayType[str]
+
+            >>> array = numpy.array(["bar"])
+            >>> enum_array = MyEnum.encode(array)
+            >>> enum_array[0] == MyEnum.bar.index
+            True
+
+        .. seealso::
+            :meth:`.EnumArray.decode` for decoding.
 
-        >>> free_lodger = HousingOccupancyStatus.free_lodger
-        >>> owner = HousingOccupancyStatus.owner
-        >>> enum_item_array = asarray([free_lodger, owner])
-        >>> encoded_array = HousingOccupancyStatus.encode(enum_item_array)
-        >>> encoded_array[0]
-        2  # Encoded value
         """
+        conditions: list[t.Array[t.DTypeBool]]
+        choices: list[int]
+
         if isinstance(array, EnumArray):
+
             return array
 
-        # String array
-        if isinstance(array, numpy.ndarray) and array.dtype.kind in {"U", "S"}:
-            array = numpy.select(
-                [array == item.name for item in cls],
-                [item.index for item in cls],
-            ).astype(ENUM_ARRAY_DTYPE)
-
-        # Enum items arrays
-        elif isinstance(array, numpy.ndarray) and array.dtype.kind == "O":
-            # Ensure we are comparing the comparable. The problem this fixes:
-            # On entering this method "cls" will generally come from
-            # variable.possible_values, while the array values may come from
-            # directly importing a module containing an Enum class. However,
-            # variables (and hence their possible_values) are loaded by a call
-            # to load_module, which gives them a different identity from the
-            # ones imported in the usual way.
-            #
-            # So, instead of relying on the "cls" passed in, we use only its
-            # name to check that the values in the array, if non-empty, are of
-            # the right type.
-            if len(array) > 0 and cls.__name__ is array[0].__class__.__name__:
-                cls = array[0].__class__
-
-            array = numpy.select(
-                [array == item for item in cls],
-                [item.index for item in cls],
-            ).astype(ENUM_ARRAY_DTYPE)
+        if (
+            isinstance(array, numpy.ndarray)
+            and array.size > 0
+            and isinstance(array.take(0), (Enum, bytes, str))
+        ):
+
+            if numpy.issubdtype(array.dtype, bytes):
+
+                array = array.astype(str)
+
+            if numpy.issubdtype(array.dtype, str):
+
+                conditions = [array == item.name for item in cls]
+
+            if numpy.issubdtype(array.dtype, cls):
+                # Ensure we are comparing the comparable.
+                #
+                # The problem this fixes:
+                #
+                # On entering this method "cls" will generally come from
+                # variable.possible_values, while the array values may come
+                # from directly importing a module containing an Enum class.
+                #
+                # However, variables (and hence their possible_values) are
+                # loaded by a call to load_module, which gives them a different
+                # identity from the ones imported in the usual way.
+                #
+                # So, instead of relying on the "cls" passed in, we use only
+                # its name to check that the values in the array, if non-empty,
+                # are of the right type.
+
+                cls = array.take(0).__class__
+                conditions = [array == item for item in cls]
+
+            choices = [item.index for item in cls]
+
+            array = numpy.select(conditions, choices).astype(ENUM_ARRAY_DTYPE)
 
         return EnumArray(array, cls)
 
diff --git a/openfisca_core/indexed_enums/enum_array.py b/openfisca_core/indexed_enums/enum_array.py
index 93c848698..1d7ceb6c6 100644
--- a/openfisca_core/indexed_enums/enum_array.py
+++ b/openfisca_core/indexed_enums/enum_array.py
@@ -1,10 +1,9 @@
 from __future__ import annotations
 
-from typing import Any, NoReturn
+from typing import NoReturn
 from typing_extensions import Self
 
 import numpy
-
 from . import types as t
 
 
@@ -13,6 +12,51 @@ class EnumArray(t.EnumArray):
 
     :class:`.Enum` arrays are encoded as :class:`int` to improve performance.
 
+    Examples:
+        >>> from openfisca_core.indexed_enums import Enum
+        >>> from openfisca_core.variables import Variable
+
+        >>> class Housing(Enum):
+        ...     OWNER = "Owner"
+        ...     TENANT = "Tenant"
+        ...     FREE_LODGER = "Free lodger"
+        ...     HOMELESS = "Homeless"
+
+        >>> array = numpy.array([1])
+        >>> enum_array = EnumArray(array, Housing)
+
+        >>> repr(EnumArray)
+        "<class 'openfisca_core.indexed_enums.enum_array.EnumArray'>"
+
+        >>> repr(enum_array)
+        '<EnumArray([<Housing.TENANT(Tenant)>])>'
+
+        >>> str(enum_array)
+        "['TENANT']"
+
+        >>> list(enum_array)
+        [1]
+
+        >>> enum_array[0]
+        1
+
+        >>> enum_array[0] in enum_array
+        True
+
+        >>> len(enum_array)
+        1
+
+        >>> enum_array = EnumArray(list(Housing), Housing)
+        >>> enum_array[Housing.TENANT.index]
+        <Housing.TENANT(Tenant)>
+
+        >>> class OccupancyStatus(Variable):
+        ...     value_type = Enum
+        ...     possible_values = Housing
+
+        >>> EnumArray(array, OccupancyStatus.possible_values)
+        <EnumArray([<Housing.TENANT(Tenant)>])>
+
     Note:
         Subclassing :class:`~numpy.ndarray` is a little tricky™. To read more
         about the :meth:`.__new__` and :meth:`.__array_finalize__` methods
@@ -40,10 +84,15 @@ def __array_finalize__(self, obj: numpy.int32 | None) -> None:
         """See comment above."""
         if obj is None:
             return
-
         self.possible_values = getattr(obj, "possible_values", None)
 
-    def __eq__(self, other: object) -> bool:
+    def __repr__(self) -> str:
+        return f"{self.__class__.__name__}({self.decode()!s})"
+
+    def __str__(self) -> str:
+        return str(self.decode_to_str())
+
+    def __eq__(self, other: object) -> bool | t.Array[t.DTypeBool]:
         """Compare equality with the item's :attr:`~.Enum.index`.
 
         When comparing to an item of :attr:`.possible_values`, use the
@@ -59,6 +108,31 @@ def __eq__(self, other: object) -> bool:
             bool: When ???
             ndarray[bool_]: When ???
 
+        Examples:
+            >>> from openfisca_core.indexed_enums import Enum
+
+            >>> class MyEnum(Enum):
+            ...     FOO = b"foo"
+            ...     BAR = b"bar"
+
+            >>> array = numpy.array([1])
+            >>> enum_array = EnumArray(array, MyEnum)
+
+            >>> enum_array == 1
+            array([ True])
+
+            >>> enum_array == [1]
+            array([ True])
+
+            >>> enum_array == [2]
+            array([False])
+
+            >>> enum_array == "1"
+            False
+
+            >>> enum_array is None
+            array([False])
+
         Note:
             This breaks the `Liskov substitution principle`_.
 
@@ -68,10 +142,9 @@ def __eq__(self, other: object) -> bool:
         """
         if other.__class__.__name__ is self.possible_values.__name__:
             return self.view(numpy.ndarray) == other.index
-
         return self.view(numpy.ndarray) == other
 
-    def __ne__(self, other: object) -> bool:
+    def __ne__(self, other: object) -> bool | t.Array[t.DTypeBool]:
         """Inequality.
 
         Args:
@@ -81,6 +154,31 @@ def __ne__(self, other: object) -> bool:
             bool: When ???
             ndarray[bool_]: When ???
 
+        Examples:
+            >>> from openfisca_core.indexed_enums import Enum
+
+            >>> class MyEnum(Enum):
+            ...     FOO = b"foo"
+            ...     BAR = b"bar"
+
+            >>> array = numpy.array([1])
+            >>> enum_array = EnumArray(array, MyEnum)
+
+            >>> enum_array != 1
+            array([False])
+
+            >>> enum_array != [1]
+            array([False])
+
+            >>> enum_array != [2]
+            array([ True])
+
+            >>> enum_array != "1"
+            True
+
+            >>> enum_array is not None
+            array([ True])
+
         Note:
             This breaks the `Liskov substitution principle`_.
 
@@ -91,14 +189,12 @@ def __ne__(self, other: object) -> bool:
         return numpy.logical_not(self == other)
 
     @staticmethod
-    def _forbidden_operation(other: Any) -> NoReturn:
+    def _forbidden_operation(other: object) -> NoReturn:
         msg = (
             "Forbidden operation. The only operations allowed on EnumArrays "
             "are '==' and '!='."
         )
-        raise TypeError(
-            msg,
-        )
+        raise TypeError(msg)
 
     __add__ = _forbidden_operation
     __mul__ = _forbidden_operation
@@ -115,15 +211,17 @@ def decode(self) -> numpy.object_:
         Returns:
             ndarray[Enum]: The items of the :obj:`.EnumArray`.
 
-        For instance:
+        Examples:
+            >>> from openfisca_core.indexed_enums import Enum
 
-        >>> enum_array = household("housing_occupancy_status", period)
-        >>> enum_array[0]
-        >>> 2  # Encoded value
-        >>> enum_array.decode()[0]
-        <HousingOccupancyStatus.free_lodger: 'Free lodger'>
+            >>> class MyEnum(Enum):
+            ...     FOO = b"foo"
+            ...     BAR = b"bar"
 
-        Decoded value: enum item
+            >>> array = numpy.array([1])
+            >>> enum_array = EnumArray(array, MyEnum)
+            >>> enum_array.decode()
+            array([<MyEnum.BAR(b'bar')>], dtype=object)
 
         """
         return numpy.select(
@@ -137,13 +235,17 @@ def decode_to_str(self) -> numpy.str_:
         Returns:
             ndarray[str_]: The string values of the :obj:`.EnumArray`.
 
-        For instance:
+        Examples:
+            >>> from openfisca_core.indexed_enums import Enum
 
-        >>> enum_array = household("housing_occupancy_status", period)
-        >>> enum_array[0]
-        >>> 2  # Encoded value
-        >>> enum_array.decode_to_str()[0]
-        'free_lodger'  # String identifier
+            >>> class MyEnum(Enum):
+            ...     FOO = b"foo"
+            ...     BAR = b"bar"
+
+            >>> array = numpy.array([1])
+            >>> enum_array = EnumArray(array, MyEnum)
+            >>> enum_array.decode_to_str()
+            array(['BAR']...)
 
         """
         return numpy.select(
@@ -151,11 +253,5 @@ def decode_to_str(self) -> numpy.str_:
             [item.name for item in self.possible_values],
         )
 
-    def __repr__(self) -> str:
-        return f"{self.__class__.__name__}({self.decode()!s})"
-
-    def __str__(self) -> str:
-        return str(self.decode_to_str())
-
 
 __all__ = ["EnumArray"]
diff --git a/openfisca_core/indexed_enums/tests/__init__.py b/openfisca_core/indexed_enums/tests/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/openfisca_core/indexed_enums/tests/test_enum.py b/openfisca_core/indexed_enums/tests/test_enum.py
new file mode 100644
index 000000000..953a5e121
--- /dev/null
+++ b/openfisca_core/indexed_enums/tests/test_enum.py
@@ -0,0 +1,153 @@
+import numpy
+
+from openfisca_core.indexed_enums import Enum
+
+
+class MyEnum(Enum):
+    """An enum…"""
+
+    foo = b"foo"
+    bar = b"bar"
+
+
+# ArrayLike["Enum"]
+
+
+def test_enum_encode_with_enum_scalar_array():
+    """Encodes when called with an enum scalar array."""
+
+    array = numpy.array(MyEnum.bar)
+    enum_array = MyEnum.encode(array)
+    assert enum_array == MyEnum.bar.index
+
+
+def test_enum_encode_with_enum_sequence():
+    """Does not encode when called with an enum sequence."""
+
+    sequence = list(MyEnum)
+    enum_array = MyEnum.encode(sequence)
+    assert enum_array[0] != MyEnum.bar.index
+
+
+def test_enum_encode_with_enum_scalar():
+    """Does not encode when called with an enum scalar."""
+
+    scalar = MyEnum.bar
+    enum_array = MyEnum.encode(scalar)
+    assert enum_array != MyEnum.bar.index
+
+
+# ArrayLike[bytes]
+
+
+def test_enum_encode_with_bytes_scalar_array():
+    """Encodes when called with a bytes scalar array."""
+
+    array = numpy.array(b"bar")
+    enum_array = MyEnum.encode(array)
+    assert enum_array == MyEnum.bar.index
+
+
+def test_enum_encode_with_bytes_sequence():
+    """Does not encode when called with a bytes sequence."""
+
+    sequence = bytearray(b"bar")
+    enum_array = MyEnum.encode(sequence)
+    assert enum_array[0] != MyEnum.bar.index
+
+
+def test_enum_encode_with_bytes_scalar():
+    """Does not encode when called with a bytes scalar."""
+
+    scalar = b"bar"
+    enum_array = MyEnum.encode(scalar)
+    assert enum_array != MyEnum.bar.index
+
+
+# ArrayLike[int]
+
+
+def test_enum_encode_with_int_scalar_array():
+    """Does not encode when called with an int scalar array (noop)."""
+
+    array = numpy.array(1)
+    enum_array = MyEnum.encode(array)
+    assert enum_array == MyEnum.bar.index
+
+
+def test_enum_encode_with_int_sequence():
+    """Does not encode when called with an int sequence (noop)."""
+
+    sequence = range(1, 2)
+    enum_array = MyEnum.encode(sequence)
+    assert enum_array[0] == MyEnum.bar.index
+
+
+def test_enum_encode_with_int_scalar():
+    """Does not encode when called with an int scalar (noop)."""
+
+    scalar = 1
+    enum_array = MyEnum.encode(scalar)
+    assert enum_array == MyEnum.bar.index
+
+
+# ArrayLike[str]
+
+
+def test_enum_encode_with_str_scalar_array():
+    """Encodes when called with a str scalar array."""
+
+    array = numpy.array("bar")
+    enum_array = MyEnum.encode(array)
+    assert enum_array == MyEnum.bar.index
+
+
+def test_enum_encode_with_str_sequence():
+    """Does not encode when called with a str sequence."""
+
+    sequence = tuple(("bar",))
+    enum_array = MyEnum.encode(sequence)
+    assert enum_array[0] != MyEnum.bar.index
+
+
+def test_enum_encode_with_str_scalar():
+    """Does not encode when called with a str scalar."""
+
+    scalar = "bar"
+    enum_array = MyEnum.encode(scalar)
+    assert enum_array != MyEnum.bar.index
+
+
+# Unsupported encodings
+
+
+def test_enum_encode_with_any_array():
+    """Does not encode when called with unsupported types (noop)."""
+
+    array = numpy.array([{"foo": "bar"}])
+    enum_array = MyEnum.encode(array)
+    assert enum_array[0] == {"foo": "bar"}
+
+
+def test_enum_encode_with_any_scalar_array():
+    """Does not encode when called with unsupported types (noop)."""
+
+    array = numpy.array(1.5)
+    enum_array = MyEnum.encode(array)
+    assert enum_array == 1.5
+
+
+def test_enum_encode_with_any_sequence():
+    """Does not encode when called with unsupported types (noop)."""
+
+    sequence = memoryview(b"bar")
+    enum_array = MyEnum.encode(sequence)
+    assert enum_array[0] == sequence[0]
+
+
+def test_enum_encode_with_anything():
+    """Does not encode when called with unsupported types (noop)."""
+
+    anything = {object()}
+    enum_array = MyEnum.encode(anything)
+    assert enum_array == anything
diff --git a/openfisca_core/indexed_enums/tests/test_enum_array.py b/openfisca_core/indexed_enums/tests/test_enum_array.py
new file mode 100644
index 000000000..5ccc5256f
--- /dev/null
+++ b/openfisca_core/indexed_enums/tests/test_enum_array.py
@@ -0,0 +1,37 @@
+import numpy
+import pytest
+
+from openfisca_core.indexed_enums import Enum, EnumArray
+
+
+class MyEnum(Enum):
+    """An enum…"""
+
+    FOO = b"foo"
+    BAR = b"bar"
+
+
+@pytest.fixture
+def enum_array():
+    """An enum array…"""
+
+    return EnumArray([numpy.array(1)], MyEnum)
+
+
+def test_enum_array_eq_operation(enum_array):
+    """The equality operation is permitted."""
+
+    assert enum_array == EnumArray([numpy.array(1)], MyEnum)
+
+
+def test_enum_array_ne_operation(enum_array):
+    """The non-equality operation is permitted."""
+
+    assert enum_array != EnumArray([numpy.array(0)], MyEnum)
+
+
+def test_enum_array_any_other_operation(enum_array):
+    """Only equality and non-equality operations are permitted."""
+
+    with pytest.raises(TypeError, match="Forbidden operation."):
+        enum_array * 1
diff --git a/openfisca_core/indexed_enums/types.py b/openfisca_core/indexed_enums/types.py
index d69eb098a..34c0ad1b0 100644
--- a/openfisca_core/indexed_enums/types.py
+++ b/openfisca_core/indexed_enums/types.py
@@ -1,3 +1,3 @@
-from openfisca_core.types import Array, DTypeEnum, Enum, EnumArray
+from openfisca_core.types import Array, DTypeBool, DTypeEnum, Enum, EnumArray
 
-__all__ = ["Array", "DTypeEnum", "Enum", "EnumArray"]
+__all__ = ["Array", "DTypeBool", "DTypeEnum", "Enum", "EnumArray"]
diff --git a/openfisca_core/types/protocols/builder.py b/openfisca_core/types/protocols/builder.py
new file mode 100644
index 000000000..7120daa53
--- /dev/null
+++ b/openfisca_core/types/protocols/builder.py
@@ -0,0 +1,37 @@
+from __future__ import annotations
+
+from typing import Iterable, Sequence, Type, TypeVar
+from typing_extensions import Protocol
+
+import abc
+
+RT = TypeVar("RT", covariant=True)
+ET = TypeVar("ET", covariant=True)
+EL = TypeVar("EL", contravariant=True)
+
+
+class Builder(Protocol[RT, ET, EL]):
+    """Base type for any model implementing a builder protocol.
+
+    Type-checking against abstractions rather than implementations helps in
+    (a) decoupling the codebse, thanks to structural subtyping, and
+    (b) documenting/enforcing the blueprints of the different OpenFisca models.
+
+    .. versionadded:: 35.7.0
+
+    """
+
+    @abc.abstractmethod
+    def __init__(self, __builder: RT, __buildee: Type[ET]) -> None: ...
+
+    @abc.abstractmethod
+    def __call__(self, __items: Iterable[EL]) -> Sequence[ET]:
+        """A concrete builder implements :meth:`.__call__`."""
+
+        ...
+
+    @abc.abstractmethod
+    def build(self, __item: EL) -> ET:
+        """A concrete builder implements :meth:`.build`."""
+
+        ...
diff --git a/openfisca_core/types/protocols/descriptor.py b/openfisca_core/types/protocols/descriptor.py
new file mode 100644
index 000000000..aa5406960
--- /dev/null
+++ b/openfisca_core/types/protocols/descriptor.py
@@ -0,0 +1,20 @@
+from typing import Any, Optional, Type, TypeVar
+from typing_extensions import Protocol
+
+T = TypeVar("T", covariant=True)
+
+
+class Descriptor(Protocol[T]):
+    """Base type for any model implementing a descriptor protocol.
+
+    Type-checking against abstractions rather than implementations helps in
+    (a) decoupling the codebse, thanks to structural subtyping, and
+    (b) documenting/enforcing the blueprints of the different OpenFisca models.
+
+    .. versionadded:: 35.7.0
+
+    """
+
+    def __get__(self, __instance: Any, __owner: Type[Any]) -> Optional[T]: ...
+
+    def __set__(self, __instance: Any, __value: Any) -> None: ...