diff --git a/src/type/__tests__/validation-test.js b/src/type/__tests__/validation-test.js
index f7f10e308a..e541ca7d2f 100644
--- a/src/type/__tests__/validation-test.js
+++ b/src/type/__tests__/validation-test.js
@@ -341,6 +341,17 @@ describe('Type System: Objects must have fields', () => {
     );
   });
 
+  it('rejects an Object type with reserved named fields', () => {
+    expect(
+      () => schemaWithFieldType(new GraphQLObjectType({
+        name: 'SomeObject',
+        fields: { __notPartOfIntrospection: { type: GraphQLString } }
+      }))
+    ).to.throw(
+      'Name "__notPartOfIntrospection" must not begin with "__", which is reserved by GraphQL introspection.'
+    );
+  });
+
   it('rejects an Object type with incorrectly typed fields', () => {
     expect(
       () => schemaWithFieldType(new GraphQLObjectType({
diff --git a/src/type/definition.js b/src/type/definition.js
index 25909da450..2cf5a61d86 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -291,7 +291,6 @@ export class GraphQLScalarType {
   _scalarConfig: GraphQLScalarTypeConfig<*, *>;
 
   constructor(config: GraphQLScalarTypeConfig<*, *>) {
-    invariant(config.name, 'Type must be named.');
     assertValidName(config.name);
     this.name = config.name;
     this.description = config.description;
@@ -400,8 +399,7 @@ export class GraphQLObjectType {
   _interfaces: Array<GraphQLInterfaceType>;
 
   constructor(config: GraphQLObjectTypeConfig<*, *>) {
-    invariant(config.name, 'Type must be named.');
-    assertValidName(config.name);
+    assertValidName(config.name, config.isIntrospection);
     this.name = config.name;
     this.description = config.description;
     if (config.isTypeOf) {
@@ -547,7 +545,8 @@ export type GraphQLObjectTypeConfig<TSource, TContext> = {
   interfaces?: Thunk<?Array<GraphQLInterfaceType>>;
   fields: Thunk<GraphQLFieldConfigMap<TSource, TContext>>;
   isTypeOf?: ?GraphQLIsTypeOfFn<TSource, TContext>;
-  description?: ?string
+  description?: ?string;
+  isIntrospection?: boolean;
 };
 
 export type GraphQLTypeResolver<TSource, TContext> = (
@@ -656,7 +655,6 @@ export class GraphQLInterfaceType {
   _fields: GraphQLFieldMap<*, *>;
 
   constructor(config: GraphQLInterfaceTypeConfig<*, *>) {
-    invariant(config.name, 'Type must be named.');
     assertValidName(config.name);
     this.name = config.name;
     this.description = config.description;
@@ -735,7 +733,6 @@ export class GraphQLUnionType {
   _possibleTypeNames: {[typeName: string]: boolean};
 
   constructor(config: GraphQLUnionTypeConfig<*, *>) {
-    invariant(config.name, 'Type must be named.');
     assertValidName(config.name);
     this.name = config.name;
     this.description = config.description;
@@ -845,7 +842,7 @@ export class GraphQLEnumType/* <T> */ {
 
   constructor(config: GraphQLEnumTypeConfig/* <T> */) {
     this.name = config.name;
-    assertValidName(config.name);
+    assertValidName(config.name, config.isIntrospection);
     this.description = config.description;
     this._values = defineEnumValues(this, config.values);
     this._enumConfig = config;
@@ -953,6 +950,7 @@ export type GraphQLEnumTypeConfig/* <T> */ = {
   name: string;
   values: GraphQLEnumValueConfigMap/* <T> */;
   description?: ?string;
+  isIntrospection?: boolean;
 };
 
 export type GraphQLEnumValueConfigMap/* <T> */ = {
@@ -1003,7 +1001,6 @@ export class GraphQLInputObjectType {
   _fields: GraphQLInputFieldMap;
 
   constructor(config: GraphQLInputObjectTypeConfig) {
-    invariant(config.name, 'Type must be named.');
     assertValidName(config.name);
     this.name = config.name;
     this.description = config.description;
diff --git a/src/type/introspection.js b/src/type/introspection.js
index fe67e97498..09e94c5fe4 100644
--- a/src/type/introspection.js
+++ b/src/type/introspection.js
@@ -28,6 +28,7 @@ import type { GraphQLField } from './definition';
 
 export const __Schema = new GraphQLObjectType({
   name: '__Schema',
+  isIntrospection: true,
   description:
     'A GraphQL Schema defines the capabilities of a GraphQL server. It ' +
     'exposes all available types and directives on the server, as well as ' +
@@ -69,6 +70,7 @@ export const __Schema = new GraphQLObjectType({
 
 export const __Directive = new GraphQLObjectType({
   name: '__Directive',
+  isIntrospection: true,
   description:
     'A Directive provides a way to describe alternate runtime execution and ' +
     'type validation behavior in a GraphQL document.' +
@@ -117,6 +119,7 @@ export const __Directive = new GraphQLObjectType({
 
 export const __DirectiveLocation = new GraphQLEnumType({
   name: '__DirectiveLocation',
+  isIntrospection: true,
   description:
     'A Directive can be adjacent to many parts of the GraphQL language, a ' +
     '__DirectiveLocation describes one such possible adjacencies.',
@@ -198,6 +201,7 @@ export const __DirectiveLocation = new GraphQLEnumType({
 
 export const __Type = new GraphQLObjectType({
   name: '__Type',
+  isIntrospection: true,
   description:
     'The fundamental unit of any GraphQL Schema is the type. There are ' +
     'many kinds of types in GraphQL as represented by the `__TypeKind` enum.' +
@@ -299,6 +303,7 @@ export const __Type = new GraphQLObjectType({
 
 export const __Field = new GraphQLObjectType({
   name: '__Field',
+  isIntrospection: true,
   description:
     'Object and Interface types are described by a list of Fields, each of ' +
     'which has a name, potentially a list of arguments, and a return type.',
@@ -320,6 +325,7 @@ export const __Field = new GraphQLObjectType({
 
 export const __InputValue = new GraphQLObjectType({
   name: '__InputValue',
+  isIntrospection: true,
   description:
     'Arguments provided to Fields or Directives and the input fields of an ' +
     'InputObject are represented as Input Values which describe their type ' +
@@ -342,6 +348,7 @@ export const __InputValue = new GraphQLObjectType({
 
 export const __EnumValue = new GraphQLObjectType({
   name: '__EnumValue',
+  isIntrospection: true,
   description:
     'One possible value for a given Enum. Enum values are unique values, not ' +
     'a placeholder for a string or numeric value. However an Enum value is ' +
@@ -369,6 +376,7 @@ export const TypeKind = {
 
 export const __TypeKind = new GraphQLEnumType({
   name: '__TypeKind',
+  isIntrospection: true,
   description: 'An enum describing what kind of type a given `__Type` is.',
   values: {
     SCALAR: {
diff --git a/src/utilities/assertValidName.js b/src/utilities/assertValidName.js
index 8413767375..cd5d8881b8 100644
--- a/src/utilities/assertValidName.js
+++ b/src/utilities/assertValidName.js
@@ -8,15 +8,29 @@
  *  of patent rights can be found in the PATENTS file in the same directory.
  */
 
-import invariant from '../jsutils/invariant';
-
-
 const NAME_RX = /^[_a-zA-Z][_a-zA-Z0-9]*$/;
 
-// Helper to assert that provided names are valid.
-export function assertValidName(name: string): void {
-  invariant(
-    NAME_RX.test(name),
-    `Names must match /^[_a-zA-Z][_a-zA-Z0-9]*$/ but "${name}" does not.`
-  );
+/**
+ * Upholds the spec rules about naming.
+ */
+export function assertValidName(
+  name: string,
+  isIntrospection?: boolean
+): void {
+  if (!name || typeof name !== 'string') {
+    throw new Error(
+      `Must be named. Unexpected name: ${name}.`
+    );
+  }
+  if (!isIntrospection && name.slice(0, 2) === '__') {
+    throw new Error(
+      `Name "${name}" must not begin with "__", which is reserved by ` +
+      'GraphQL introspection.'
+    );
+  }
+  if (!NAME_RX.test(name)) {
+    throw new Error(
+      `Names must match /^[_a-zA-Z][_a-zA-Z0-9]*$/ but "${name}" does not.`
+    );
+  }
 }