diff --git a/src/language/__tests__/schema-kitchen-sink.graphql b/src/language/__tests__/schema-kitchen-sink.graphql
index f94f47c8e5..20fe9b48b6 100644
--- a/src/language/__tests__/schema-kitchen-sink.graphql
+++ b/src/language/__tests__/schema-kitchen-sink.graphql
@@ -65,6 +65,8 @@ extend union Feed @onUnion
 
 scalar CustomScalar
 
+scalar StringEncodedCustomScalar as String
+
 scalar AnnotatedScalar @onScalar
 
 extend scalar CustomScalar @onScalar
diff --git a/src/language/__tests__/schema-parser-test.js b/src/language/__tests__/schema-parser-test.js
index 1c9fe0f144..9eb879ba05 100644
--- a/src/language/__tests__/schema-parser-test.js
+++ b/src/language/__tests__/schema-parser-test.js
@@ -712,6 +712,7 @@ type Hello {
         {
           kind: 'ScalarTypeDefinition',
           name: nameNode('Hello', { start: 7, end: 12 }),
+          type: undefined,
           directives: [],
           loc: { start: 0, end: 12 },
         },
diff --git a/src/language/__tests__/schema-printer-test.js b/src/language/__tests__/schema-printer-test.js
index 1bacb8e586..b6324e5b1c 100644
--- a/src/language/__tests__/schema-printer-test.js
+++ b/src/language/__tests__/schema-printer-test.js
@@ -109,6 +109,8 @@ describe('Printer', () => {
 
       scalar CustomScalar
 
+      scalar StringEncodedCustomScalar as String
+
       scalar AnnotatedScalar @onScalar
 
       extend scalar CustomScalar @onScalar
diff --git a/src/language/ast.js b/src/language/ast.js
index bec5ae3d57..395fd78132 100644
--- a/src/language/ast.js
+++ b/src/language/ast.js
@@ -448,6 +448,7 @@ export type ScalarTypeDefinitionNode = {
   +loc?: Location,
   +description?: StringValueNode,
   +name: NameNode,
+  +type?: NamedTypeNode,
   +directives?: $ReadOnlyArray<DirectiveNode>,
 };
 
diff --git a/src/language/parser.js b/src/language/parser.js
index afa5d6ecab..a60ba1a2c7 100644
--- a/src/language/parser.js
+++ b/src/language/parser.js
@@ -890,18 +890,21 @@ function parseOperationTypeDefinition(
 }
 
 /**
- * ScalarTypeDefinition : Description? scalar Name Directives[Const]?
+ * ScalarTypeDefinition : Description? scalar Name ScalarOfType? Directives[Const]?
+ * ScalarOfType : as NamedType
  */
 function parseScalarTypeDefinition(lexer: Lexer<*>): ScalarTypeDefinitionNode {
   const start = lexer.token;
   const description = parseDescription(lexer);
   expectKeyword(lexer, 'scalar');
   const name = parseName(lexer);
+  const type = skipKeyword(lexer, 'as') ? parseNamedType(lexer) : undefined;
   const directives = parseDirectives(lexer, true);
   return {
     kind: SCALAR_TYPE_DEFINITION,
     description,
     name,
+    type,
     directives,
     loc: loc(lexer, start),
   };
@@ -1503,10 +1506,24 @@ function expect(lexer: Lexer<*>, kind: string): Token {
 }
 
 /**
- * If the next token is a keyword with the given value, return that token after
+ * If the next token is a keyword with the given value, return true after
  * advancing the lexer. Otherwise, do not change the parser state and return
  * false.
  */
+function skipKeyword(lexer: Lexer<*>, value: string): boolean {
+  const token = lexer.token;
+  const match = token.kind === TokenKind.NAME && token.value === value;
+  if (match) {
+    lexer.advance();
+  }
+  return match;
+}
+
+/**
+ * If the next token is a keyword with the given value, return that token after
+ * advancing the lexer. Otherwise, do not change the parser state and throw
+ * an error.
+ */
 function expectKeyword(lexer: Lexer<*>, value: string): Token {
   const token = lexer.token;
   if (token.kind === TokenKind.NAME && token.value === value) {
diff --git a/src/language/printer.js b/src/language/printer.js
index 10bb63595a..a7d94e9f69 100644
--- a/src/language/printer.js
+++ b/src/language/printer.js
@@ -110,12 +110,14 @@ const printDocASTReducer = {
 
   OperationTypeDefinition: ({ operation, type }) => operation + ': ' + type,
 
-  ScalarTypeDefinition: ({ description, name, directives }) =>
+  ScalarTypeDefinition: ({ description, name, type, directives }) =>
     join(
-      [description, join(['scalar', name, join(directives, ' ')], ' ')],
+      [
+        description,
+        join(['scalar', name, wrap('as ', type), join(directives, ' ')], ' '),
+      ],
       '\n',
     ),
-
   ObjectTypeDefinition: ({
     description,
     name,
diff --git a/src/language/visitor.js b/src/language/visitor.js
index 31937548ee..0148c96aae 100644
--- a/src/language/visitor.js
+++ b/src/language/visitor.js
@@ -101,7 +101,7 @@ export const QueryDocumentKeys = {
   SchemaDefinition: ['directives', 'operationTypes'],
   OperationTypeDefinition: ['type'],
 
-  ScalarTypeDefinition: ['description', 'name', 'directives'],
+  ScalarTypeDefinition: ['description', 'name', 'type', 'directives'],
   ObjectTypeDefinition: [
     'description',
     'name',
diff --git a/src/type/__tests__/introspection-test.js b/src/type/__tests__/introspection-test.js
index e47aea1081..b1ad489ced 100644
--- a/src/type/__tests__/introspection-test.js
+++ b/src/type/__tests__/introspection-test.js
@@ -1307,7 +1307,9 @@ describe('Introspection', () => {
             'An enum describing what kind of type a given `__Type` is.',
           enumValues: [
             {
-              description: 'Indicates this type is a scalar.',
+              description:
+                'Indicates this type is a scalar. ' +
+                '`ofType` may represent how this scalar is serialized.',
               name: 'SCALAR',
             },
             {
diff --git a/src/type/definition.js b/src/type/definition.js
index 2e54f99ca2..f17bc90adf 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -450,12 +450,14 @@ export class GraphQLScalarType {
   name: string;
   description: ?string;
   astNode: ?ScalarTypeDefinitionNode;
+  ofType: ?GraphQLScalarType;
 
   _scalarConfig: GraphQLScalarTypeConfig<*, *>;
 
   constructor(config: GraphQLScalarTypeConfig<*, *>): void {
     this.name = config.name;
     this.description = config.description;
+    this.ofType = config.ofType || null;
     this.astNode = config.astNode;
     this._scalarConfig = config;
     invariant(typeof config.name === 'string', 'Must provide name.');
@@ -478,12 +480,14 @@ export class GraphQLScalarType {
   // Serializes an internal value to include in a response.
   serialize(value: mixed): mixed {
     const serializer = this._scalarConfig.serialize;
-    return serializer(value);
+    const serialized = serializer(value);
+    return this.ofType ? this.ofType.serialize(serialized) : serialized;
   }
 
   // Parses an externally provided value to use as an input.
   parseValue(value: mixed): mixed {
-    const parser = this._scalarConfig.parseValue;
+    const parser =
+      this._scalarConfig.parseValue || (this.ofType && this.ofType.parseValue);
     if (isInvalid(value)) {
       return undefined;
     }
@@ -492,7 +496,9 @@ export class GraphQLScalarType {
 
   // Parses an externally provided literal value to use as an input.
   parseLiteral(valueNode: ValueNode, variables: ?ObjMap<mixed>): mixed {
-    const parser = this._scalarConfig.parseLiteral;
+    const parser =
+      this._scalarConfig.parseLiteral ||
+      (this.ofType && this.ofType.parseLiteral);
     return parser
       ? parser(valueNode, variables)
       : valueFromASTUntyped(valueNode, variables);
@@ -513,6 +519,7 @@ GraphQLScalarType.prototype.toJSON = GraphQLScalarType.prototype.inspect =
 export type GraphQLScalarTypeConfig<TInternal, TExternal> = {
   name: string,
   description?: ?string,
+  ofType?: ?GraphQLScalarType,
   astNode?: ?ScalarTypeDefinitionNode,
   serialize: (value: mixed) => ?TExternal,
   parseValue?: (value: mixed) => ?TInternal,
diff --git a/src/type/introspection.js b/src/type/introspection.js
index 759f4b8f2b..907368c01a 100644
--- a/src/type/introspection.js
+++ b/src/type/introspection.js
@@ -207,8 +207,8 @@ export const __Type = new GraphQLObjectType({
     'The fundamental unit of any GraphQL Schema is the type. There are ' +
     'many kinds of types in GraphQL as represented by the `__TypeKind` enum.' +
     '\n\nDepending on the kind of a type, certain fields describe ' +
-    'information about that type. Scalar types provide no information ' +
-    'beyond a name and description, while Enum types provide their values. ' +
+    'information about that type. Scalar types provide a name, description ' +
+    'and how they serialize, while Enum types provide their possible values. ' +
     'Object and Interface types provide the fields they describe. Abstract ' +
     'types, Union and Interface, provide the Object types possible ' +
     'at runtime. List and NonNull types compose other types.',
@@ -381,7 +381,9 @@ export const __TypeKind = new GraphQLEnumType({
   values: {
     SCALAR: {
       value: TypeKind.SCALAR,
-      description: 'Indicates this type is a scalar.',
+      description:
+        'Indicates this type is a scalar. ' +
+        '`ofType` may represent how this scalar is serialized.',
     },
     OBJECT: {
       value: TypeKind.OBJECT,
diff --git a/src/type/validate.js b/src/type/validate.js
index bf6ec8315a..eab5779851 100644
--- a/src/type/validate.js
+++ b/src/type/validate.js
@@ -8,6 +8,7 @@
  */
 
 import {
+  isScalarType,
   isObjectType,
   isInterfaceType,
   isUnionType,
@@ -19,6 +20,7 @@ import {
   isOutputType,
 } from './definition';
 import type {
+  GraphQLScalarType,
   GraphQLObjectType,
   GraphQLInterfaceType,
   GraphQLUnionType,
@@ -30,6 +32,7 @@ import type { GraphQLDirective } from './directives';
 import { isIntrospectionType } from './introspection';
 import { isSchema } from './schema';
 import type { GraphQLSchema } from './schema';
+import { isSpecifiedScalarType } from './scalars';
 import find from '../jsutils/find';
 import invariant from '../jsutils/invariant';
 import { GraphQLError } from '../error/GraphQLError';
@@ -239,7 +242,10 @@ function validateTypes(context: SchemaValidationContext): void {
     // Ensure they are named correctly.
     validateName(context, type);
 
-    if (isObjectType(type)) {
+    if (isScalarType(type)) {
+      // Ensure ofType is a built-in scalar
+      validateScalarOfType(context, type);
+    } else if (isObjectType(type)) {
       // Ensure fields are valid
       validateFields(context, type);
 
@@ -261,6 +267,20 @@ function validateTypes(context: SchemaValidationContext): void {
   });
 }
 
+function validateScalarOfType(
+  context: SchemaValidationContext,
+  type: GraphQLScalarType,
+): void {
+  const ofType = type.ofType;
+  if (ofType && !isSpecifiedScalarType(ofType)) {
+    context.reportError(
+      `Type ${type.name} may only be described in terms of a built-in scalar ` +
+        `type. However ${ofType.name} is not a built-in scalar type.`,
+      type.astNode,
+    );
+  }
+}
+
 function validateFields(
   context: SchemaValidationContext,
   type: GraphQLObjectType | GraphQLInterfaceType,
diff --git a/src/utilities/__tests__/schemaPrinter-test.js b/src/utilities/__tests__/schemaPrinter-test.js
index 96a6a21104..9748251deb 100644
--- a/src/utilities/__tests__/schemaPrinter-test.js
+++ b/src/utilities/__tests__/schemaPrinter-test.js
@@ -498,8 +498,17 @@ describe('Type System Printer', () => {
   });
 
   it('Custom Scalar', () => {
+    const EvenType = new GraphQLScalarType({
+      name: 'Even',
+      ofType: GraphQLInt,
+      serialize(value) {
+        return value % 2 === 1 ? value : null;
+      },
+    });
+
     const OddType = new GraphQLScalarType({
       name: 'Odd',
+      // No ofType in this test case.
       serialize(value) {
         return value % 2 === 1 ? value : null;
       },
@@ -508,6 +517,7 @@ describe('Type System Printer', () => {
     const Root = new GraphQLObjectType({
       name: 'Root',
       fields: {
+        even: { type: EvenType },
         odd: { type: OddType },
       },
     });
@@ -519,9 +529,12 @@ describe('Type System Printer', () => {
         query: Root
       }
 
+      scalar Even as Int
+
       scalar Odd
 
       type Root {
+        even: Even
         odd: Odd
       }
     `);
@@ -780,10 +793,10 @@ describe('Type System Printer', () => {
       types in GraphQL as represented by the \`__TypeKind\` enum.
 
       Depending on the kind of a type, certain fields describe information about that
-      type. Scalar types provide no information beyond a name and description, while
-      Enum types provide their values. Object and Interface types provide the fields
-      they describe. Abstract types, Union and Interface, provide the Object types
-      possible at runtime. List and NonNull types compose other types.
+      type. Scalar types provide a name, description and how they serialize, while
+      Enum types provide their possible values. Object and Interface types provide the
+      fields they describe. Abstract types, Union and Interface, provide the Object
+      types possible at runtime. List and NonNull types compose other types.
       """
       type __Type {
         kind: __TypeKind!
@@ -799,7 +812,9 @@ describe('Type System Printer', () => {
 
       """An enum describing what kind of type a given \`__Type\` is."""
       enum __TypeKind {
-        """Indicates this type is a scalar."""
+        """
+        Indicates this type is a scalar. \`ofType\` may represent how this scalar is serialized.
+        """
         SCALAR
 
         """
@@ -1000,10 +1015,10 @@ describe('Type System Printer', () => {
       # types in GraphQL as represented by the \`__TypeKind\` enum.
       #
       # Depending on the kind of a type, certain fields describe information about that
-      # type. Scalar types provide no information beyond a name and description, while
-      # Enum types provide their values. Object and Interface types provide the fields
-      # they describe. Abstract types, Union and Interface, provide the Object types
-      # possible at runtime. List and NonNull types compose other types.
+      # type. Scalar types provide a name, description and how they serialize, while
+      # Enum types provide their possible values. Object and Interface types provide the
+      # fields they describe. Abstract types, Union and Interface, provide the Object
+      # types possible at runtime. List and NonNull types compose other types.
       type __Type {
         kind: __TypeKind!
         name: String
@@ -1018,7 +1033,7 @@ describe('Type System Printer', () => {
 
       # An enum describing what kind of type a given \`__Type\` is.
       enum __TypeKind {
-        # Indicates this type is a scalar.
+        # Indicates this type is a scalar. \`ofType\` may represent how this scalar is serialized.
         SCALAR
 
         # Indicates this type is an object. \`fields\` and \`interfaces\` are valid fields.
diff --git a/src/utilities/buildASTSchema.js b/src/utilities/buildASTSchema.js
index 16ed9a99f4..27690f6882 100644
--- a/src/utilities/buildASTSchema.js
+++ b/src/utilities/buildASTSchema.js
@@ -442,6 +442,10 @@ export class ASTDefinitionBuilder {
     return new GraphQLScalarType({
       name: def.name.value,
       description: getDescription(def, this._options),
+      // Note: While this could make assertions to get the correctly typed
+      // value, that would throw immediately while type system validation
+      // with validateSchema() will produce more actionable results.
+      ofType: def.type && (this.buildType(def.type): any),
       astNode: def,
       serialize: value => value,
     });
diff --git a/src/utilities/buildClientSchema.js b/src/utilities/buildClientSchema.js
index cc883b3a88..a8ee367249 100644
--- a/src/utilities/buildClientSchema.js
+++ b/src/utilities/buildClientSchema.js
@@ -26,6 +26,7 @@ import {
   GraphQLEnumType,
   GraphQLInputObjectType,
   assertNullableType,
+  assertScalarType,
   assertObjectType,
   assertInterfaceType,
 } from '../type/definition';
@@ -179,6 +180,11 @@ export function buildClientSchema(
     return assertInterfaceType(type);
   }
 
+  function getScalarType(typeRef: IntrospectionTypeRef): GraphQLScalarType {
+    const type = getType(typeRef);
+    return assertScalarType(type);
+  }
+
   // Given a type's introspection result, construct the correct
   // GraphQLType instance.
   function buildType(type: IntrospectionType): GraphQLNamedType {
@@ -208,9 +214,13 @@ export function buildClientSchema(
   function buildScalarDef(
     scalarIntrospection: IntrospectionScalarType,
   ): GraphQLScalarType {
+    const ofType = scalarIntrospection.ofType
+      ? getScalarType(scalarIntrospection.ofType)
+      : undefined;
     return new GraphQLScalarType({
       name: scalarIntrospection.name,
       description: scalarIntrospection.description,
+      ofType,
       serialize: value => value,
     });
   }
diff --git a/src/utilities/findBreakingChanges.js b/src/utilities/findBreakingChanges.js
index b5787df2d4..61a3ea2185 100644
--- a/src/utilities/findBreakingChanges.js
+++ b/src/utilities/findBreakingChanges.js
@@ -161,6 +161,18 @@ export function findTypesThatChangedKind(
           `${typeName} changed from ` +
           `${typeKindName(oldType)} to ${typeKindName(newType)}.`,
       });
+    } else if (isScalarType(oldType) && isScalarType(newType)) {
+      const oldOfType = oldType.ofType;
+      const newOfType = newType.ofType;
+      if (oldOfType && newOfType && oldOfType !== newOfType) {
+        breakingChanges.push({
+          type: BreakingChangeType.TYPE_CHANGED_KIND,
+          description:
+            `${typeName} changed from ` +
+            `${typeKindName(oldType)} serialized as ${oldOfType.name} ` +
+            `to ${typeKindName(newType)} serialized as ${newOfType.name}.`,
+        });
+      }
     }
   });
   return breakingChanges;
diff --git a/src/utilities/introspectionQuery.js b/src/utilities/introspectionQuery.js
index 4071250998..26ca0867a5 100644
--- a/src/utilities/introspectionQuery.js
+++ b/src/utilities/introspectionQuery.js
@@ -150,6 +150,7 @@ export type IntrospectionScalarType = {
   +kind: 'SCALAR',
   +name: string,
   +description?: ?string,
+  +ofType?: ?IntrospectionNamedTypeRef<IntrospectionScalarType>,
 };
 
 export type IntrospectionObjectType = {
diff --git a/src/utilities/schemaPrinter.js b/src/utilities/schemaPrinter.js
index 568274e148..84249933ce 100644
--- a/src/utilities/schemaPrinter.js
+++ b/src/utilities/schemaPrinter.js
@@ -170,7 +170,8 @@ export function printType(type: GraphQLNamedType, options?: Options): string {
 }
 
 function printScalar(type: GraphQLScalarType, options): string {
-  return printDescription(options, type) + `scalar ${type.name}`;
+  const ofType = type.ofType ? ` as ${type.ofType.name}` : '';
+  return printDescription(options, type) + `scalar ${type.name}${ofType}`;
 }
 
 function printObject(type: GraphQLObjectType, options): string {