diff --git a/src/cereggii/_cereggii.pyi b/src/cereggii/_cereggii.pyi
index bc7221ac..0a5752f4 100644
--- a/src/cereggii/_cereggii.pyi
+++ b/src/cereggii/_cereggii.pyi
@@ -1,8 +1,8 @@
-from collections.abc import Iterator
+from collections.abc import Iterable, Iterator
 from typing import Callable, NewType, SupportsComplex, SupportsFloat, SupportsInt
 
 Key = NewType("Key", object)
-Value = NewType("Value", object | None)
+Value = NewType("Value", object)
 Cancel = NewType("Cancel", object)
 
 Number = SupportsInt | SupportsFloat | SupportsComplex
@@ -137,29 +137,36 @@ class AtomicDict:
 
         :returns: the input :param:`batch` dictionary, with substituted values.
         """
-    # def aggregate(self, iterator: Iterator[tuple[Key, Value]], aggregation: Callable[[Key, Value, Value], Value])
-    # -> None:
-    #     """
-    #     Aggregate the values in this dictionary with those found in ``iterator``, as computed by ``aggregation``.
-    #
-    #     The ``aggregation`` parameter expects a function that takes as input a key, the value currently stored
-    #     in the dictionary, and the new value from ``iterator``, and then returns the aggregated value.
-    #
-    #     For instance, to aggregate counts::
-    #
-    #         d = AtomicDict()
-    #
-    #         it = [
-    #             ("red", 1),
-    #             ("green", 3),
-    #             ("blue", 40),
-    #             ("red", 1),
-    #         ]
-    #
-    #         d.aggregate(it, lambda key, current, new:
-    #             new if current is cereggii.NOT_FOUND else current + new
-    #         )
-    #     """
+    def reduce(
+        self,
+        iterable: Iterable[tuple[Key, Value]],
+        aggregate: Callable[[Key, Value, Value], Value],
+        chunk_size: int = 0,
+    ) -> None:
+        """
+        Aggregate the values in this dictionary with those found in ``iterable``, as computed by ``aggregate``.
+
+        The ``aggregate`` function takes as input a key, the value currently stored in the dictionary,
+        and the new value from ``iterator``. It returns the aggregated value.
+
+        For instance, to aggregate some simple counts::
+
+            d = AtomicDict()
+
+            data = [
+                ("red", 1),
+                ("green", 42),
+                ("blue", 3),
+                ("red", 5),
+            ]
+
+            def count(key, current, new):
+                if current is cereggii.NOT_FOUND:
+                    return new
+                return current + new
+
+            d.reduce(data, count)
+        """
     def compact(self) -> None: ...
     def debug(self) -> dict: ...
     def rehash(self, o: object) -> int: ...
diff --git a/src/cereggii/atomic_dict/insert.c b/src/cereggii/atomic_dict/insert.c
index 55f1a8d9..54fe0b80 100644
--- a/src/cereggii/atomic_dict/insert.c
+++ b/src/cereggii/atomic_dict/insert.c
@@ -488,3 +488,227 @@ AtomicDict_SetItem(AtomicDict *self, PyObject *key, PyObject *value)
     fail:
     return -1;
 }
+
+
+#define REDUCE_FLUSH_CHUNK_SIZE 64
+
+
+static inline int
+reduce_flush(AtomicDict *self, PyObject *local_buffer, PyObject *aggregate)
+{
+    AtomicDict_Meta *meta = NULL;
+    PyObject **keys = NULL;
+    PyObject **expected = NULL;
+    PyObject **desired = NULL;
+
+    keys = PyMem_RawMalloc(REDUCE_FLUSH_CHUNK_SIZE * sizeof(PyObject *));
+    if (keys == NULL)
+        goto fail;
+
+    expected = PyMem_RawMalloc(REDUCE_FLUSH_CHUNK_SIZE * sizeof(PyObject *));
+    if (expected == NULL)
+        goto fail;
+
+    desired = PyMem_RawMalloc(REDUCE_FLUSH_CHUNK_SIZE * sizeof(PyObject *));
+    if (desired == NULL)
+        goto fail;
+
+    Py_ssize_t pos = 0;
+
+    get_meta:
+    meta = (AtomicDict_Meta *) AtomicRef_Get(self->metadata);
+    if (meta == NULL)
+        goto fail;
+
+    for (;;) {
+        int num_items = 0;
+        int done = 0;
+
+        for (int i = 0; i < REDUCE_FLUSH_CHUNK_SIZE; ++i) {
+            PyObject *values;
+            if (PyDict_Next(local_buffer, &pos, &keys[num_items], &values)) {
+                expected[num_items] = PyTuple_GetItem(values, 0);
+                desired[num_items] = PyTuple_GetItem(values, 1);
+
+                if (expected[num_items] == NULL || desired[num_items] == NULL)
+                    goto fail;
+
+                num_items++;
+            } else {
+                done = 1;
+                break;
+            }
+        }
+
+        for (int i = 0; i < num_items; ++i) {
+            // prefetch index
+            Py_hash_t hash = PyObject_Hash(keys[i]);
+            if (hash == -1)
+                goto fail;
+
+            __builtin_prefetch(AtomicDict_IndexAddressOf(
+                AtomicDict_Distance0Of(hash, meta),
+                meta
+            ));
+        }
+
+        for (int i = 0; i < num_items; ++i) {
+            PyObject *result = NULL;
+            PyObject *new = desired[i];
+
+            retry:
+            result = AtomicDict_CompareAndSet(self, keys[i], expected[i], new);
+
+            if (result == NULL)
+                goto fail;
+
+            if (result == EXPECTATION_FAILED) {
+                PyObject *current = AtomicDict_GetItemOrDefault(self, keys[i], NOT_FOUND);
+                if (current == NULL)
+                    goto fail;
+
+                expected[i] = current;
+                new = PyObject_CallFunctionObjArgs(aggregate, keys[i], expected[i], desired[i], NULL);
+                if (new == NULL)
+                    goto fail;
+
+                goto retry;
+            }
+        }
+
+        if (done)
+            break;
+
+
+        if (meta->new_gen_metadata != NULL)
+            goto get_meta;
+    }
+
+    PyMem_RawFree(keys);
+    PyMem_RawFree(expected);
+    PyMem_RawFree(desired);
+    Py_DECREF(meta);
+    return 0;
+
+    fail:
+    if (keys != NULL) {
+        PyMem_RawFree(keys);
+    }
+    if (expected != NULL) {
+        PyMem_RawFree(expected);
+    }
+    if (desired != NULL) {
+        PyMem_RawFree(desired);
+    }
+    Py_XDECREF(meta);
+    return -1;
+}
+
+
+int
+AtomicDict_Reduce(AtomicDict *self, PyObject *iterable, PyObject *aggregate, int chunk_size)
+{
+    PyObject *local_buffer = NULL;
+    PyObject *item = NULL;
+
+    if (!PyCallable_Check(aggregate)) {
+        PyErr_Format(PyExc_TypeError, "%R is not callable.", aggregate);
+        goto fail;
+    }
+
+    if (chunk_size != 0) {
+        PyErr_SetString(PyExc_ValueError, "only supporting chunk_size=0 at the moment.");
+        goto fail;
+    }
+
+    local_buffer = PyDict_New();
+    if (local_buffer == NULL)
+        goto fail;
+
+    PyObject *iterator = PyObject_GetIter(iterable);
+
+    if (iterator == NULL) {
+        PyErr_Format(PyExc_TypeError, "%R is not iterable.", iterable);
+        goto fail;
+    }
+
+    while ((item = PyIter_Next(iterator))) {
+        PyObject *key = NULL;
+        PyObject *value = NULL;
+        PyObject *current = NULL;
+        PyObject *expected = NULL;
+        PyObject *desired = NULL;
+
+        if (!PyTuple_CheckExact(item)) {
+            PyErr_Format(PyExc_TypeError, "type(%R) != tuple", item);
+            goto fail;
+        }
+
+        if (PyTuple_Size(item) != 2) {
+            PyErr_Format(PyExc_TypeError, "len(%R) != 2", item);
+            goto fail;
+        }
+
+        key = PyTuple_GetItem(item, 0);
+        value = PyTuple_GetItem(item, 1);
+        if (PyDict_Contains(local_buffer, key)) {
+            current = PyDict_GetItem(local_buffer, key);
+            expected = PyTuple_GetItem(current, 0);
+            current = PyTuple_GetItem(current, 1);
+        } else {
+            expected = NOT_FOUND;
+            current = expected;
+        }
+
+        desired = PyObject_CallFunctionObjArgs(aggregate, key, current, value, NULL);
+        if (desired == NULL)
+            goto fail;
+
+        PyObject *tuple = PyTuple_Pack(2, expected, desired);
+        if (tuple == NULL)
+            goto fail;
+
+        if (PyDict_SetItem(local_buffer, key, tuple) < 0)
+            goto fail;
+
+        Py_DECREF(item);
+    }
+
+    Py_DECREF(iterator);
+
+    if (PyErr_Occurred()) {
+        goto fail;
+    }
+
+    reduce_flush(self, local_buffer, aggregate);
+
+    Py_DECREF(local_buffer);
+    return 0;
+
+    fail:
+    Py_XDECREF(local_buffer);
+    Py_XDECREF(item);
+    return -1;
+}
+
+PyObject *
+AtomicDict_Reduce_callable(AtomicDict *self, PyObject *args, PyObject *kwargs)
+{
+    PyObject *iterable = NULL;
+    PyObject *aggregate = NULL;
+    int chunk_size = 0;
+
+    char *kw_list[] = {"iterable", "aggregate", "chunk_size", NULL};
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "OO|i", kw_list, &iterable, &aggregate, &chunk_size))
+        goto fail;
+
+    int res = AtomicDict_Reduce(self, iterable, aggregate, chunk_size);
+    if (res < 0)
+        goto fail;
+
+    Py_RETURN_NONE;
+
+    fail:
+    return NULL;
+}
diff --git a/src/cereggii/cereggii.c b/src/cereggii/cereggii.c
index d4842fb7..179ce595 100644
--- a/src/cereggii/cereggii.c
+++ b/src/cereggii/cereggii.c
@@ -222,6 +222,7 @@ static PyMethodDef AtomicDict_methods[] = {
     {"fast_iter",       (PyCFunction) AtomicDict_FastIter,                METH_VARARGS | METH_KEYWORDS, NULL},
     {"compare_and_set", (PyCFunction) AtomicDict_CompareAndSet_callable,  METH_VARARGS | METH_KEYWORDS, NULL},
     {"batch_getitem",   (PyCFunction) AtomicDict_BatchGetItem,            METH_VARARGS | METH_KEYWORDS, NULL},
+    {"reduce",          (PyCFunction) AtomicDict_Reduce_callable,         METH_VARARGS | METH_KEYWORDS, NULL},
     {NULL}
 };
 
diff --git a/src/include/atomic_dict.h b/src/include/atomic_dict.h
index 910b41b6..883be576 100644
--- a/src/include/atomic_dict.h
+++ b/src/include/atomic_dict.h
@@ -48,6 +48,10 @@ PyObject *AtomicDict_CompareAndSet(AtomicDict *self, PyObject *key, PyObject *ex
 
 PyObject *AtomicDict_CompareAndSet_callable(AtomicDict *self, PyObject *args, PyObject *kwargs);
 
+int AtomicDict_Reduce(AtomicDict *self, PyObject *iterable, PyObject *aggregate, int chunk_size);
+
+PyObject *AtomicDict_Reduce_callable(AtomicDict *self, PyObject *args, PyObject *kwargs);
+
 int AtomicDict_Compact(AtomicDict *self);
 
 PyObject *AtomicDict_Compact_callable(AtomicDict *self);
diff --git a/tests/test_atomic_dict.py b/tests/test_atomic_dict.py
index 47ee5255..7d0d8675 100644
--- a/tests/test_atomic_dict.py
+++ b/tests/test_atomic_dict.py
@@ -719,3 +719,25 @@ def test_len():
 
     assert len(d) == 10
     assert len(d) == 10  # test twice for len_dirty
+
+
+def test_reduce():
+    d = AtomicDict()
+
+    data = [
+        ("red", 1),
+        ("green", 42),
+        ("blue", 3),
+        ("red", 5),
+    ]
+
+    def count(key, current, new):  # noqa: ARG001
+        if current is cereggii.NOT_FOUND:
+            return new
+        return current + new
+
+    d.reduce(data, count)
+
+    assert d["red"] == 6
+    assert d["green"] == 42
+    assert d["blue"] == 3