diff --git a/src/utilities/__tests__/buildASTSchema-test.js b/src/utilities/__tests__/buildASTSchema-test.js
index c92ed212d3..7d06723e9e 100644
--- a/src/utilities/__tests__/buildASTSchema-test.js
+++ b/src/utilities/__tests__/buildASTSchema-test.js
@@ -394,6 +394,137 @@ describe('Schema Builder', () => {
     expect(output).to.equal(body);
   });
 
+  it('Specifying Union type using __typename', async () => {
+    const schema = buildSchema(`
+      schema {
+        query: Root
+      }
+
+      type Root {
+        fruits: [Fruit]
+      }
+
+      union Fruit = Apple | Banana
+
+      type Apple {
+        color: String
+      }
+
+      type Banana {
+        length: Int
+      }
+    `);
+
+    const query = `
+      {
+        fruits {
+          ... on Apple {
+            color
+          }
+          ... on Banana {
+            length
+          }
+        }
+      }
+    `;
+
+    const root = {
+      fruits: [
+        {
+          color: 'green',
+          __typename: 'Apple',
+        },
+        {
+          length: 5,
+          __typename: 'Banana',
+        }
+      ]
+    };
+
+    expect(await graphql(schema, query, root)).to.deep.equal({
+      data: {
+        fruits: [
+          {
+            color: 'green',
+          },
+          {
+            length: 5,
+          }
+        ]
+      }
+    });
+  });
+
+  it('Specifying Interface type using __typename', async () => {
+    const schema = buildSchema(`
+      schema {
+        query: Root
+      }
+
+      type Root {
+        characters: [Character]
+      }
+
+      interface Character {
+        name: String!
+      }
+
+      type Human implements Character {
+        name: String!
+        totalCredits: Int
+      }
+
+      type Droid implements Character {
+        name: String!
+        primaryFunction: String
+      }
+    `);
+
+    const query = `
+      {
+        characters {
+          name
+          ... on Human {
+            totalCredits
+          }
+          ... on Droid {
+            primaryFunction
+          }
+        }
+      }
+    `;
+
+    const root = {
+      characters: [
+        {
+          name: 'Han Solo',
+          totalCredits: 10,
+          __typename: 'Human',
+        },
+        {
+          name: 'R2-D2',
+          primaryFunction: 'Astromech',
+          __typename: 'Droid',
+        }
+      ]
+    };
+
+    expect(await graphql(schema, query, root)).to.deep.equal({
+      data: {
+        characters: [
+          {
+            name: 'Han Solo',
+            totalCredits: 10,
+          },
+          {
+            name: 'R2-D2',
+            primaryFunction: 'Astromech',
+          }
+        ]
+      }
+    });
+  });
+
   it('Custom Scalar', () => {
     const body = dedent`
       schema {
diff --git a/src/utilities/buildASTSchema.js b/src/utilities/buildASTSchema.js
index f1038b3955..3280a4a12b 100644
--- a/src/utilities/buildASTSchema.js
+++ b/src/utilities/buildASTSchema.js
@@ -11,6 +11,7 @@
 import invariant from '../jsutils/invariant';
 import keyValMap from '../jsutils/keyValMap';
 import { valueFromAST } from './valueFromAST';
+import { resolveTypeForGeneratedSchema } from './resolveTypeForGeneratedSchema';
 import { TokenKind } from '../language/lexer';
 import { parse } from '../language/parser';
 import type { Source } from '../language/source';
@@ -398,7 +399,7 @@ export function buildASTSchema(ast: DocumentNode): GraphQLSchema {
       description: getDescription(def),
       fields: () => makeFieldDefMap(def),
       astNode: def,
-      resolveType: cannotExecuteSchema,
+      resolveType: resolveTypeForGeneratedSchema,
     });
   }
 
@@ -424,7 +425,7 @@ export function buildASTSchema(ast: DocumentNode): GraphQLSchema {
       name: def.name.value,
       description: getDescription(def),
       types: def.types.map(t => produceObjectType(t)),
-      resolveType: cannotExecuteSchema,
+      resolveType: resolveTypeForGeneratedSchema,
       astNode: def,
     });
   }
@@ -516,9 +517,3 @@ function leadingSpaces(str) {
   }
   return i;
 }
-
-function cannotExecuteSchema() {
-  throw new Error(
-    'Generated Schema cannot use Interface or Union types for execution.'
-  );
-}
diff --git a/src/utilities/extendSchema.js b/src/utilities/extendSchema.js
index 70776ed31c..bd97c624b3 100644
--- a/src/utilities/extendSchema.js
+++ b/src/utilities/extendSchema.js
@@ -16,6 +16,7 @@ import {
   getDeprecationReason,
 } from './buildASTSchema';
 import { valueFromAST } from './valueFromAST';
+import { resolveTypeForGeneratedSchema } from './resolveTypeForGeneratedSchema';
 import { GraphQLError } from '../error/GraphQLError';
 import { GraphQLSchema } from '../type/schema';
 
@@ -479,7 +480,7 @@ export function extendSchema(
       description: getDescription(typeNode),
       fields: () => buildFieldMap(typeNode),
       astNode: typeNode,
-      resolveType: cannotExecuteExtendedSchema,
+      resolveType: resolveTypeForGeneratedSchema,
     });
   }
 
@@ -489,7 +490,7 @@ export function extendSchema(
       description: getDescription(typeNode),
       types: typeNode.types.map(getObjectTypeFromAST),
       astNode: typeNode,
-      resolveType: cannotExecuteExtendedSchema,
+      resolveType: resolveTypeForGeneratedSchema,
     });
   }
 
@@ -608,9 +609,3 @@ export function extendSchema(
     return getOutputTypeFromAST(typeNode);
   }
 }
-
-function cannotExecuteExtendedSchema() {
-  throw new Error(
-    'Extended Schema cannot use Interface or Union types for execution.'
-  );
-}
diff --git a/src/utilities/resolveTypeForGeneratedSchema.js b/src/utilities/resolveTypeForGeneratedSchema.js
new file mode 100644
index 0000000000..18d2999ea6
--- /dev/null
+++ b/src/utilities/resolveTypeForGeneratedSchema.js
@@ -0,0 +1,18 @@
+// @flow
+
+import type { GraphQLTypeResolver } from '../type/definition';
+
+export const resolveTypeForGeneratedSchema: GraphQLTypeResolver<any, *> =
+function (value: any): string {
+  if (
+    value &&
+    typeof value === 'object' &&
+    typeof value.__typename === 'string'
+  ) {
+    return value.__typename;
+  }
+  throw new Error(
+    'To resolve Interface or Union types for a generated Schema the result ' +
+    'must have a __typename property containing the name of the actual type.'
+  );
+};