diff --git a/src/__tests__/starWarsQueryTests.js b/src/__tests__/starWarsQueryTests.js
index f838926fd8..180e12b3af 100644
--- a/src/__tests__/starWarsQueryTests.js
+++ b/src/__tests__/starWarsQueryTests.js
@@ -181,7 +181,7 @@ describe('Star Wars Query Tests', () => {
           name: 'Luke Skywalker'
         }
       };
-      const result = await graphql(StarWarsSchema, query, null, params);
+      const result = await graphql(StarWarsSchema, query, null, null, params);
       expect(result).to.deep.equal({ data: expected });
     });
 
@@ -201,7 +201,7 @@ describe('Star Wars Query Tests', () => {
           name: 'Han Solo'
         }
       };
-      const result = await graphql(StarWarsSchema, query, null, params);
+      const result = await graphql(StarWarsSchema, query, null, null, params);
       expect(result).to.deep.equal({ data: expected });
     });
 
@@ -219,7 +219,7 @@ describe('Star Wars Query Tests', () => {
       const expected = {
         human: null
       };
-      const result = await graphql(StarWarsSchema, query, null, params);
+      const result = await graphql(StarWarsSchema, query, null, null, params);
       expect(result).to.deep.equal({ data: expected });
     });
   });
diff --git a/src/execution/__tests__/executor.js b/src/execution/__tests__/executor.js
index 7df25e0c08..f15277b1ad 100644
--- a/src/execution/__tests__/executor.js
+++ b/src/execution/__tests__/executor.js
@@ -137,7 +137,7 @@ describe('Execute: Handles basic execution tasks', () => {
     });
 
     expect(
-      await execute(schema, ast, data, { size: 100 }, 'Example')
+      await execute(schema, ast, data, null, { size: 100 }, 'Example')
     ).to.deep.equal(expected);
   });
 
@@ -427,7 +427,7 @@ describe('Execute: Handles basic execution tasks', () => {
       })
     });
 
-    const result = await execute(schema, ast, data, null, 'OtherExample');
+    const result = await execute(schema, ast, data, null, null, 'OtherExample');
 
     expect(result).to.deep.equal({ data: { second: 'b' } });
   });
@@ -481,7 +481,9 @@ describe('Execute: Handles basic execution tasks', () => {
       })
     });
 
-    expect(() => execute(schema, ast, data, null, 'UnknownExample')).to.throw(
+    expect(() =>
+      execute(schema, ast, data, null, null, 'UnknownExample')
+    ).to.throw(
       'Unknown operation named "UnknownExample".'
     );
   });
@@ -511,7 +513,7 @@ describe('Execute: Handles basic execution tasks', () => {
       })
     });
 
-    const queryResult = await execute(schema, ast, data, {}, 'Q');
+    const queryResult = await execute(schema, ast, data, null, {}, 'Q');
 
     expect(queryResult).to.deep.equal({ data: { a: 'b' } });
   });
@@ -535,7 +537,7 @@ describe('Execute: Handles basic execution tasks', () => {
       })
     });
 
-    const mutationResult = await execute(schema, ast, data, {}, 'M');
+    const mutationResult = await execute(schema, ast, data, null, {}, 'M');
 
     expect(mutationResult).to.deep.equal({ data: { c: 'd' } });
   });
@@ -559,7 +561,7 @@ describe('Execute: Handles basic execution tasks', () => {
       })
     });
 
-    const subscriptionResult = await execute(schema, ast, data, {}, 'S');
+    const subscriptionResult = await execute(schema, ast, data, null, {}, 'S');
 
     expect(subscriptionResult).to.deep.equal({ data: { a: 'b' } });
   });
@@ -644,7 +646,7 @@ describe('Execute: Handles basic execution tasks', () => {
       }),
     });
 
-    const queryResult = await execute(schema, ast, data, {}, 'Q');
+    const queryResult = await execute(schema, ast, data, null, {}, 'Q');
 
     expect(queryResult).to.deep.equal({ data: { a: 'b' } });
   });
diff --git a/src/execution/__tests__/union-interface.js b/src/execution/__tests__/union-interface.js
index 0156e20c73..5797ca07f5 100644
--- a/src/execution/__tests__/union-interface.js
+++ b/src/execution/__tests__/union-interface.js
@@ -349,6 +349,7 @@ describe('Execute: Union and intersection types', () => {
   });
 
   it('gets execution info in resolver', async () => {
+    let encounteredContext;
     let encounteredSchema;
     let encounteredRootValue;
 
@@ -357,9 +358,10 @@ describe('Execute: Union and intersection types', () => {
       fields: {
         name: { type: GraphQLString }
       },
-      resolveType(obj, { schema: infoSchema, rootValue: infoRootValue }) {
-        encounteredSchema = infoSchema;
-        encounteredRootValue = infoRootValue;
+      resolveType(obj, context, { schema: _schema, rootValue }) {
+        encounteredContext = context;
+        encounteredSchema = _schema;
+        encounteredRootValue = rootValue;
         return PersonType2;
       }
     });
@@ -379,14 +381,17 @@ describe('Execute: Union and intersection types', () => {
 
     const john2 = new Person('John', [], [ liz ]);
 
+    const context = { authToken: '123abc' };
+
     const ast = parse('{ name, friends { name } }');
 
     expect(
-      await execute(schema2, ast, john2)
+      await execute(schema2, ast, john2, context)
     ).to.deep.equal({
       data: { name: 'John', friends: [ { name: 'Liz' } ] }
     });
 
+    expect(encounteredContext).to.equal(context);
     expect(encounteredSchema).to.equal(schema2);
     expect(encounteredRootValue).to.equal(john2);
   });
diff --git a/src/execution/__tests__/variables.js b/src/execution/__tests__/variables.js
index d4d67e1477..01b649766a 100644
--- a/src/execution/__tests__/variables.js
+++ b/src/execution/__tests__/variables.js
@@ -204,7 +204,7 @@ describe('Execute: Handles inputs', () => {
 
       it('executes with complex input', async () => {
         const params = { input: { a: 'foo', b: [ 'bar' ], c: 'baz' } };
-        const result = await execute(schema, ast, null, params);
+        const result = await execute(schema, ast, null, null, params);
 
         return expect(result).to.deep.equal({
           data: {
@@ -231,7 +231,7 @@ describe('Execute: Handles inputs', () => {
 
       it('properly parses single value to list', async () => {
         const params = { input: { a: 'foo', b: 'bar', c: 'baz' } };
-        const result = await execute(schema, ast, null, params);
+        const result = await execute(schema, ast, null, null, params);
 
         return expect(result).to.deep.equal({
           data: {
@@ -242,7 +242,7 @@ describe('Execute: Handles inputs', () => {
 
       it('executes with complex scalar input', async () => {
         const params = { input: { c: 'foo', d: 'SerializedValue' } };
-        const result = await execute(schema, ast, null, params);
+        const result = await execute(schema, ast, null, null, params);
 
         return expect(result).to.deep.equal({
           data: {
@@ -256,7 +256,7 @@ describe('Execute: Handles inputs', () => {
 
         let caughtError;
         try {
-          execute(schema, ast, null, params);
+          execute(schema, ast, null, null, params);
         } catch (error) {
           caughtError = error;
         }
@@ -275,7 +275,7 @@ describe('Execute: Handles inputs', () => {
 
         let caughtError;
         try {
-          execute(schema, ast, null, params);
+          execute(schema, ast, null, null, params);
         } catch (error) {
           caughtError = error;
         }
@@ -293,7 +293,7 @@ describe('Execute: Handles inputs', () => {
 
         let caughtError;
         try {
-          execute(schema, ast, null, params);
+          execute(schema, ast, null, null, params);
         } catch (error) {
           caughtError = error;
         }
@@ -317,7 +317,7 @@ describe('Execute: Handles inputs', () => {
 
         let caughtError;
         try {
-          execute(schema, nestedAst, null, params);
+          execute(schema, nestedAst, null, null, params);
         } catch (error) {
           caughtError = error;
         }
@@ -339,7 +339,7 @@ describe('Execute: Handles inputs', () => {
 
         let caughtError;
         try {
-          execute(schema, ast, null, params);
+          execute(schema, ast, null, null, params);
         } catch (error) {
           caughtError = error;
         }
@@ -411,7 +411,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { value: null })
+        await execute(schema, ast, null, null, { value: null })
       ).to.deep.equal({
         data: {
           fieldWithNullableStringInput: null
@@ -428,7 +428,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { value: 'a' })
+        await execute(schema, ast, null, null, { value: 'a' })
       ).to.deep.equal({
         data: {
           fieldWithNullableStringInput: '"a"'
@@ -484,7 +484,7 @@ describe('Execute: Handles inputs', () => {
 
       let caughtError;
       try {
-        execute(schema, ast, null, { value: null });
+        execute(schema, ast, null, null, { value: null });
       } catch (error) {
         caughtError = error;
       }
@@ -505,7 +505,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { value: 'a' })
+        await execute(schema, ast, null, null, { value: 'a' })
       ).to.deep.equal({
         data: {
           fieldWithNonNullableStringInput: '"a"'
@@ -554,7 +554,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { input: null })
+        await execute(schema, ast, null, null, { input: null })
       ).to.deep.equal({
         data: {
           list: null
@@ -571,7 +571,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { input: [ 'A' ] })
+        await execute(schema, ast, null, null, { input: [ 'A' ] })
       ).to.deep.equal({
         data: {
           list: '["A"]'
@@ -588,7 +588,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { input: [ 'A', null, 'B' ] })
+        await execute(schema, ast, null, null, { input: [ 'A', null, 'B' ] })
       ).to.deep.equal({
         data: {
           list: '["A",null,"B"]'
@@ -606,7 +606,7 @@ describe('Execute: Handles inputs', () => {
 
       let caughtError;
       try {
-        execute(schema, ast, null, { input: null });
+        execute(schema, ast, null, null, { input: null });
       } catch (error) {
         caughtError = error;
       }
@@ -627,7 +627,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { input: [ 'A' ] })
+        await execute(schema, ast, null, null, { input: [ 'A' ] })
       ).to.deep.equal({
         data: {
           nnList: '["A"]'
@@ -644,7 +644,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { input: [ 'A', null, 'B' ] })
+        await execute(schema, ast, null, null, { input: [ 'A', null, 'B' ] })
       ).to.deep.equal({
         data: {
           nnList: '["A",null,"B"]'
@@ -661,7 +661,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { input: null })
+        await execute(schema, ast, null, null, { input: null })
       ).to.deep.equal({
         data: {
           listNN: null
@@ -678,7 +678,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { input: [ 'A' ] })
+        await execute(schema, ast, null, null, { input: [ 'A' ] })
       ).to.deep.equal({
         data: {
           listNN: '["A"]'
@@ -697,7 +697,7 @@ describe('Execute: Handles inputs', () => {
 
       let caughtError;
       try {
-        execute(schema, ast, null, vars);
+        execute(schema, ast, null, null, vars);
       } catch (error) {
         caughtError = error;
       }
@@ -720,7 +720,7 @@ describe('Execute: Handles inputs', () => {
 
       let caughtError;
       try {
-        execute(schema, ast, null, { input: null });
+        execute(schema, ast, null, null, { input: null });
       } catch (error) {
         caughtError = error;
       }
@@ -741,7 +741,7 @@ describe('Execute: Handles inputs', () => {
       const ast = parse(doc);
 
       return expect(
-        await execute(schema, ast, null, { input: [ 'A' ] })
+        await execute(schema, ast, null, null, { input: [ 'A' ] })
       ).to.deep.equal({
         data: {
           nnListNN: '["A"]'
@@ -760,7 +760,7 @@ describe('Execute: Handles inputs', () => {
 
       let caughtError;
       try {
-        execute(schema, ast, null, vars);
+        execute(schema, ast, null, null, vars);
       } catch (error) {
         caughtError = error;
       }
@@ -784,7 +784,7 @@ describe('Execute: Handles inputs', () => {
 
       let caughtError;
       try {
-        execute(schema, ast, null, vars);
+        execute(schema, ast, null, null, vars);
       } catch (error) {
         caughtError = error;
       }
@@ -808,7 +808,7 @@ describe('Execute: Handles inputs', () => {
 
       let caughtError;
       try {
-        execute(schema, ast, null, vars);
+        execute(schema, ast, null, null, vars);
       } catch (error) {
         caughtError = error;
       }
diff --git a/src/execution/execute.js b/src/execution/execute.js
index 3d20d1be03..e51555b246 100644
--- a/src/execution/execute.js
+++ b/src/execution/execute.js
@@ -30,6 +30,7 @@ import type {
   GraphQLLeafType,
   GraphQLAbstractType,
   GraphQLFieldDefinition,
+  GraphQLFieldResolveFn,
   GraphQLResolveInfo,
 } from '../type/definition';
 import { GraphQLSchema } from '../type/schema';
@@ -83,6 +84,7 @@ type ExecutionContext = {
   schema: GraphQLSchema;
   fragments: {[key: string]: FragmentDefinition};
   rootValue: mixed;
+  contextValue: mixed;
   operation: OperationDefinition;
   variableValues: {[key: string]: mixed};
   errors: Array<GraphQLError>;
@@ -110,6 +112,7 @@ export function execute(
   schema: GraphQLSchema,
   documentAST: Document,
   rootValue?: mixed,
+  contextValue?: mixed,
   variableValues?: ?{[key: string]: mixed},
   operationName?: ?string
 ): Promise<ExecutionResult> {
@@ -126,6 +129,7 @@ export function execute(
     schema,
     documentAST,
     rootValue,
+    contextValue,
     variableValues,
     operationName
   );
@@ -163,6 +167,7 @@ function buildExecutionContext(
   schema: GraphQLSchema,
   documentAST: Document,
   rootValue: mixed,
+  contextValue: mixed,
   rawVariableValues: ?{[key: string]: mixed},
   operationName: ?string
 ): ExecutionContext {
@@ -203,9 +208,16 @@ function buildExecutionContext(
     operation.variableDefinitions || [],
     rawVariableValues || {}
   );
-  const exeContext: ExecutionContext =
-    { schema, fragments, rootValue, operation, variableValues, errors };
-  return exeContext;
+
+  return {
+    schema,
+    fragments,
+    rootValue,
+    contextValue,
+    operation,
+    variableValues,
+    errors
+  };
 }
 
 /**
@@ -533,7 +545,12 @@ function resolveField(
     exeContext.variableValues
   );
 
-  // The resolve function's optional third argument is a collection of
+  // The resolve function's optional third argument is a context value that
+  // is provided to every resolve function within an execution. It is commonly
+  // used to represent an authenticated user, or request-specific caches.
+  const context = exeContext.contextValue;
+
+  // The resolve function's optional fourth argument is a collection of
   // information about the current execution state.
   const info: GraphQLResolveInfo = {
     fieldName,
@@ -549,7 +566,7 @@ function resolveField(
 
   // Get the resolve function, regardless of if its result is normal
   // or abrupt (error).
-  const result = resolveOrError(resolveFn, source, args, info);
+  const result = resolveOrError(resolveFn, source, args, context, info);
 
   return completeValueCatchingError(
     exeContext,
@@ -562,18 +579,15 @@ function resolveField(
 
 // Isolates the "ReturnOrAbrupt" behavior to not de-opt the `resolveField`
 // function. Returns the result of resolveFn or the abrupt-return Error object.
-function resolveOrError<T>(
-  resolveFn: (
-    source: mixed,
-    args: { [key: string]: mixed },
-    info: GraphQLResolveInfo
-  ) => T,
+function resolveOrError(
+  resolveFn: GraphQLFieldResolveFn,
   source: mixed,
   args: { [key: string]: mixed },
+  context: mixed,
   info: GraphQLResolveInfo
-): Error | T {
+): Error | mixed {
   try {
-    return resolveFn(source, args, info);
+    return resolveFn(source, args, context, info);
   } catch (error) {
     // Sometimes a non-error is thrown, wrap it as an Error for a
     // consistent interface.
@@ -800,8 +814,8 @@ function completeAbstractValue(
   result: mixed
 ): mixed {
   const runtimeType = returnType.resolveType ?
-    returnType.resolveType(result, info) :
-    defaultResolveTypeFn(result, info, returnType);
+    returnType.resolveType(result, exeContext.contextValue, info) :
+    defaultResolveTypeFn(result, exeContext.contextValue, info, returnType);
 
   if (!runtimeType) {
     return null;
@@ -838,7 +852,8 @@ function completeObjectValue(
   // If there is an isTypeOf predicate function, call it with the
   // current result. If isTypeOf returns false, then raise an error rather
   // than continuing execution.
-  if (returnType.isTypeOf && !returnType.isTypeOf(result, info)) {
+  if (returnType.isTypeOf &&
+      !returnType.isTypeOf(result, exeContext.contextValue, info)) {
     throw new GraphQLError(
       `Expected value of type "${returnType}" but got: ${result}.`,
       fieldASTs
@@ -871,13 +886,14 @@ function completeObjectValue(
  */
 function defaultResolveTypeFn(
   value: mixed,
+  context: mixed,
   info: GraphQLResolveInfo,
   abstractType: GraphQLAbstractType
 ): ?GraphQLObjectType {
   const possibleTypes = info.schema.getPossibleTypes(abstractType);
   for (let i = 0; i < possibleTypes.length; i++) {
     const type = possibleTypes[i];
-    if (typeof type.isTypeOf === 'function' && type.isTypeOf(value, info)) {
+    if (type.isTypeOf && type.isTypeOf(value, context, info)) {
       return type;
     }
   }
@@ -889,7 +905,7 @@ function defaultResolveTypeFn(
  * and returns it as the result, or if it's a function, returns the result
  * of calling that function.
  */
-function defaultResolveFn(source: any, args, { fieldName }) {
+function defaultResolveFn(source: any, args, context, { fieldName }) {
   // ensure source is a value for which property access is acceptable.
   if (typeof source === 'object' || typeof source === 'function') {
     const property = source[fieldName];
diff --git a/src/graphql.js b/src/graphql.js
index cdc3bdda9b..4e4a764c73 100644
--- a/src/graphql.js
+++ b/src/graphql.js
@@ -44,6 +44,7 @@ export function graphql(
   schema: GraphQLSchema,
   requestString: string,
   rootValue?: mixed,
+  contextValue?: mixed,
   variableValues?: ?{[key: string]: mixed},
   operationName?: ?string
 ): Promise<GraphQLResult> {
@@ -59,6 +60,7 @@ export function graphql(
           schema,
           documentAST,
           rootValue,
+          contextValue,
           variableValues,
           operationName
         )
diff --git a/src/type/__tests__/enumType.js b/src/type/__tests__/enumType.js
index 75f4800f5d..7c55f514fc 100644
--- a/src/type/__tests__/enumType.js
+++ b/src/type/__tests__/enumType.js
@@ -172,6 +172,7 @@ describe('Type System: Enum Values', () => {
         schema,
         'query test($color: Color!) { colorEnum(fromEnum: $color) }',
         null,
+        null,
         { color: 'BLUE' }
       )
     ).to.deep.equal({
@@ -187,6 +188,7 @@ describe('Type System: Enum Values', () => {
         schema,
         'mutation x($color: Color!) { favoriteEnum(color: $color) }',
         null,
+        null,
         { color: 'GREEN' }
       )
     ).to.deep.equal({
@@ -202,6 +204,7 @@ describe('Type System: Enum Values', () => {
         schema,
         'subscription x($color: Color!) { subscribeToEnum(color: $color) }',
         null,
+        null,
         { color: 'GREEN' }
       )
     ).to.deep.equal({
@@ -217,6 +220,7 @@ describe('Type System: Enum Values', () => {
         schema,
         'query test($color: Color!) { colorEnum(fromEnum: $color) }',
         null,
+        null,
         { color: 2 }
       )
     ).to.deep.equal({
@@ -235,6 +239,7 @@ describe('Type System: Enum Values', () => {
         schema,
         'query test($color: String!) { colorEnum(fromEnum: $color) }',
         null,
+        null,
         { color: 'BLUE' }
       )
     ).to.deep.equal({
@@ -253,6 +258,7 @@ describe('Type System: Enum Values', () => {
         schema,
         'query test($color: Int!) { colorEnum(fromEnum: $color) }',
         null,
+        null,
         { color: 2 }
       )
     ).to.deep.equal({
diff --git a/src/type/definition.js b/src/type/definition.js
index 64dcb952b1..70f0bf2a6e 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -456,17 +456,20 @@ type GraphQLFieldConfigMapThunk = () => GraphQLFieldConfigMap;
 
 export type GraphQLTypeResolveFn = (
   value: mixed,
+  context: mixed,
   info: GraphQLResolveInfo
 ) => ?GraphQLObjectType
 
 export type GraphQLIsTypeOfFn = (
   value: mixed,
+  context: mixed,
   info: GraphQLResolveInfo
 ) => boolean
 
 export type GraphQLFieldResolveFn = (
   source: mixed,
   args: {[argName: string]: mixed},
+  context: mixed,
   info: GraphQLResolveInfo
 ) => mixed
 
diff --git a/src/type/introspection.js b/src/type/introspection.js
index 61c7b1cb82..d760dac70f 100644
--- a/src/type/introspection.js
+++ b/src/type/introspection.js
@@ -218,7 +218,7 @@ export const __Type = new GraphQLObjectType({
     },
     possibleTypes: {
       type: new GraphQLList(new GraphQLNonNull(__Type)),
-      resolve(type, args, { schema }) {
+      resolve(type, args, context, { schema }) {
         if (type instanceof GraphQLInterfaceType ||
             type instanceof GraphQLUnionType) {
           return schema.getPossibleTypes(type);
@@ -385,7 +385,7 @@ export const SchemaMetaFieldDef: GraphQLFieldDefinition = {
   type: new GraphQLNonNull(__Schema),
   description: 'Access the current type schema of this server.',
   args: [],
-  resolve: (source, args, { schema }) => schema
+  resolve: (source, args, context, { schema }) => schema
 };
 
 export const TypeMetaFieldDef: GraphQLFieldDefinition = {
@@ -395,7 +395,7 @@ export const TypeMetaFieldDef: GraphQLFieldDefinition = {
   args: [
     { name: 'name', type: new GraphQLNonNull(GraphQLString) }
   ],
-  resolve: (source, { name }: { name: string }, { schema }) =>
+  resolve: (source, { name }: { name: string }, context, { schema }) =>
     schema.getType(name)
 };
 
@@ -404,5 +404,5 @@ export const TypeNameMetaFieldDef: GraphQLFieldDefinition = {
   type: new GraphQLNonNull(GraphQLString),
   description: 'The name of the current Object type at runtime.',
   args: [],
-  resolve: (source, args, { parentType }) => parentType.name
+  resolve: (source, args, context, { parentType }) => parentType.name
 };
diff --git a/src/utilities/__tests__/buildClientSchema.js b/src/utilities/__tests__/buildClientSchema.js
index fcfb49e705..b9ede38c61 100644
--- a/src/utilities/__tests__/buildClientSchema.js
+++ b/src/utilities/__tests__/buildClientSchema.js
@@ -672,6 +672,7 @@ describe('Type System: build schema from introspection', () => {
       clientSchema,
       'query NoNo($v: CustomScalar) { foo(custom1: 123, custom2: $v) }',
       { foo: 'bar' },
+      null,
       { v: 'baz' }
     );
     expect(result).to.containSubset({