diff --git a/.flowconfig b/.flowconfig
index 4f071a065d..d5a9b456ec 100644
--- a/.flowconfig
+++ b/.flowconfig
@@ -1,4 +1,5 @@
 [ignore]
+.*/packages/.*/lib
 
 [include]
 
@@ -10,6 +11,7 @@ lib/types.js
 [options]
 strip_root=true
 suppress_comment= \\(.\\|\n\\)*\\$FlowFixMe
+suppress_comment= \\(.\\|\n\\)*\\$FlowIssue
 
 [version]
-0.20.1
+0.21.0
diff --git a/package.json b/package.json
index 881f3270ac..4478c7b231 100644
--- a/package.json
+++ b/package.json
@@ -14,7 +14,7 @@
     "derequire": "^2.0.2",
     "es5-shim": "^4.1.7",
     "eslint": "1.8.0",
-    "flow-bin": "^0.20.0",
+    "flow-bin": "^0.21.0",
     "fs-readdir-recursive": "^0.1.2",
     "gulp": "^3.9.0",
     "gulp-babel": "^5.3.0",
diff --git a/packages/babel-core/src/helpers/normalize-ast.js b/packages/babel-core/src/helpers/normalize-ast.js
index d2f86c22e8..cd8881a9fd 100644
--- a/packages/babel-core/src/helpers/normalize-ast.js
+++ b/packages/babel-core/src/helpers/normalize-ast.js
@@ -1,4 +1,4 @@
-/* @noflow */
+/* @flow */
 
 import * as t from "babel-types";
 
diff --git a/packages/babel-core/src/transformation/file/index.js b/packages/babel-core/src/transformation/file/index.js
index c89545c5bc..201f54be4c 100644
--- a/packages/babel-core/src/transformation/file/index.js
+++ b/packages/babel-core/src/transformation/file/index.js
@@ -537,7 +537,7 @@ export default class File extends Store {
     }
   }
 
-  makeResult({ code, map, ast, ignored } /*: BabelFileResult */): BabelFileResult {
+  makeResult({ code, map, ast, ignored }: BabelFileResult): BabelFileResult {
     let result = {
       metadata: null,
       options:  this.opts,
diff --git a/packages/babel-core/src/transformation/pipeline.js b/packages/babel-core/src/transformation/pipeline.js
index 642dcd8c43..cbaf7fc32b 100644
--- a/packages/babel-core/src/transformation/pipeline.js
+++ b/packages/babel-core/src/transformation/pipeline.js
@@ -1,17 +1,19 @@
-/* @noflow */
+/* @flow */
+/* global BabelFileResult */
+/* global BabelFileMetadata */
 
 import normalizeAst from "../helpers/normalize-ast";
 import Plugin from "./plugin";
 import File from "./file";
 
 export default class Pipeline {
-  lint(code: string, opts?: Object = {}) {
+  lint(code: string, opts?: Object = {}): BabelFileResult {
     opts.code = false;
     opts.mode = "lint";
     return this.transform(code, opts);
   }
 
-  pretransform(code: string, opts?: Object) {
+  pretransform(code: string, opts?: Object): BabelFileResult {
     let file = new File(opts, this);
     return file.wrap(code, function () {
       file.addCode(code);
@@ -20,7 +22,7 @@ export default class Pipeline {
     });
   }
 
-  transform(code: string, opts?: Object) {
+  transform(code: string, opts?: Object): BabelFileResult {
     let file = new File(opts, this);
     return file.wrap(code, function () {
       file.addCode(code);
@@ -29,7 +31,7 @@ export default class Pipeline {
     });
   }
 
-  analyse(code: string, opts: Object = {}, visitor?) {
+  analyse(code: string, opts: Object = {}, visitor?: Object): ?BabelFileMetadata {
     opts.code = false;
     if (visitor) {
       opts.plugins = opts.plugins || [];
@@ -38,7 +40,7 @@ export default class Pipeline {
     return this.transform(code, opts).metadata;
   }
 
-  transformFromAst(ast, code: string, opts: Object) {
+  transformFromAst(ast: Object, code: string, opts: Object): BabelFileResult {
     ast = normalizeAst(ast);
 
     let file = new File(opts, this);
diff --git a/packages/babel-core/src/util.js b/packages/babel-core/src/util.js
index 9e5f46f1d2..8c1c1b1d80 100644
--- a/packages/babel-core/src/util.js
+++ b/packages/babel-core/src/util.js
@@ -1,4 +1,4 @@
-/* @noflow */
+/* @flow */
 
 import escapeRegExp from "lodash/string/escapeRegExp";
 import startsWith from "lodash/string/startsWith";
@@ -16,7 +16,7 @@ export { inherits, inspect } from "util";
  * Test if a filename ends with a compilable extension.
  */
 
-export function canCompile(filename: string, altExts?: Array<string>) {
+export function canCompile(filename: string, altExts?: Array<string>): boolean {
   let exts = altExts || canCompile.EXTENSIONS;
   let ext = path.extname(filename);
   return contains(exts, ext);
diff --git a/packages/babel-generator/src/buffer.js b/packages/babel-generator/src/buffer.js
index e0e63bcd7d..af8f91ac01 100644
--- a/packages/babel-generator/src/buffer.js
+++ b/packages/babel-generator/src/buffer.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 import type Position from "./position";
 import repeating from "repeating";
 import trimRight from "trim-right";
@@ -21,17 +23,19 @@ export default class Buffer {
     this.last = "";
   }
 
+  printedCommentStarts: Object;
   parenPushNewlineState: ?Object;
-  buf: string;
   position: Position;
   _indent: number;
   format: Object;
+  buf: string;
+  last: string;
 
   /**
    * Description
    */
 
-  catchUp(node) {
+  catchUp(node: Object) {
     // catch up to this nodes newline if we're behind
     if (node.loc && this.format.retainLines && this.buf) {
       while (this.position.line < node.loc.start.line) {
@@ -44,7 +48,7 @@ export default class Buffer {
    * Get the current trimmed buffer.
    */
 
-  get() {
+  get(): string {
     return trimRight(this.buf);
   }
 
@@ -52,7 +56,7 @@ export default class Buffer {
    * Get the current indent.
    */
 
-  getIndent() {
+  getIndent(): string {
     if (this.format.compact || this.format.concise) {
       return "";
     } else {
@@ -64,7 +68,7 @@ export default class Buffer {
    * Get the current indent size.
    */
 
-  indentSize() {
+  indentSize(): number {
     return this.getIndent().length;
   }
 
@@ -222,7 +226,7 @@ export default class Buffer {
 
   _removeSpacesAfterLastNewline() {
     let lastNewlineIndex = this.buf.lastIndexOf("\n");
-    if (lastNewlineIndex >= 0 && this.buf.trimRight().length <= lastNewlineIndex) {
+    if (lastNewlineIndex >= 0 && this.get().length <= lastNewlineIndex) {
       this.buf = this.buf.substring(0, lastNewlineIndex + 1);
       this.last = "\n";
     }
@@ -251,7 +255,7 @@ export default class Buffer {
    * Push a string to the buffer.
    */
 
-  _push(str) {
+  _push(str: string): void {
     // see startTerminatorless() instance method
     let parenPushNewlineState = this.parenPushNewlineState;
     if (parenPushNewlineState) {
@@ -296,12 +300,12 @@ export default class Buffer {
    * Test if a character is last in the buffer.
    */
 
-  isLast(cha: string) {
+  isLast(cha: string): boolean {
     if (this.format.compact) return false;
     return this._isLast(cha);
   }
 
-  _isLast(cha: string) {
+  _isLast(cha: string): boolean {
     let last = this.last;
 
     if (Array.isArray(cha)) {
diff --git a/packages/babel-generator/src/generators/jsx.js b/packages/babel-generator/src/generators/jsx.js
index 7a926bb512..310a26209f 100644
--- a/packages/babel-generator/src/generators/jsx.js
+++ b/packages/babel-generator/src/generators/jsx.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 export function JSXAttribute(node: Object) {
   this.print(node.name, node);
   if (node.value) {
diff --git a/packages/babel-generator/src/generators/statements.js b/packages/babel-generator/src/generators/statements.js
index 596983e009..7e8a117616 100644
--- a/packages/babel-generator/src/generators/statements.js
+++ b/packages/babel-generator/src/generators/statements.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 import repeating from "repeating";
 import * as t from "babel-types";
 
diff --git a/packages/babel-generator/src/node/whitespace.js b/packages/babel-generator/src/node/whitespace.js
index 19cba52455..bed5500954 100644
--- a/packages/babel-generator/src/node/whitespace.js
+++ b/packages/babel-generator/src/node/whitespace.js
@@ -1,8 +1,15 @@
+/* @noflow */
+
 import isBoolean from "lodash/lang/isBoolean";
 import each from "lodash/collection/each";
 import map from "lodash/collection/map";
 import * as t from "babel-types";
 
+type WhitespaceObject = {
+  before?: boolean,
+  after?: boolean
+};
+
 /**
  * Crawl a node to test if it contains a CallExpression, a Function, or a Helper.
  *
@@ -63,7 +70,7 @@ exports.nodes = {
    * Test if AssignmentExpression needs whitespace.
    */
 
-  AssignmentExpression(node) {
+  AssignmentExpression(node: Object): ?WhitespaceObject {
     let state = crawl(node.right);
     if ((state.hasCall && state.hasHelper) || state.hasFunction) {
       return {
@@ -77,7 +84,7 @@ exports.nodes = {
    * Test if SwitchCase needs whitespace.
    */
 
-  SwitchCase(node, parent) {
+  SwitchCase(node: Object, parent: Object): ?WhitespaceObject {
     return {
       before: node.consequent.length || parent.cases[0] === node
     };
@@ -87,7 +94,7 @@ exports.nodes = {
    * Test if LogicalExpression needs whitespace.
    */
 
-  LogicalExpression(node) {
+  LogicalExpression(node: Object): ?WhitespaceObject {
     if (t.isFunction(node.left) || t.isFunction(node.right)) {
       return {
         after: true
@@ -99,7 +106,7 @@ exports.nodes = {
    * Test if Literal needs whitespace.
    */
 
-  Literal(node) {
+  Literal(node: Object): ?WhitespaceObject {
     if (node.value === "use strict") {
       return {
         after: true
@@ -111,7 +118,7 @@ exports.nodes = {
    * Test if CallExpression needs whitespace.
    */
 
-  CallExpression(node) {
+  CallExpression(node: Object): ?WhitespaceObject {
     if (t.isFunction(node.callee) || isHelper(node)) {
       return {
         before: true,
@@ -124,7 +131,7 @@ exports.nodes = {
    * Test if VariableDeclaration needs whitespace.
    */
 
-  VariableDeclaration(node) {
+  VariableDeclaration(node: Object): ?WhitespaceObject {
     for (let i = 0; i < node.declarations.length; i++) {
       let declar = node.declarations[i];
 
@@ -147,7 +154,7 @@ exports.nodes = {
    * Test if IfStatement needs whitespace.
    */
 
-  IfStatement(node) {
+  IfStatement(node: Object): ?WhitespaceObject {
     if (t.isBlockStatement(node.consequent)) {
       return {
         before: true,
@@ -163,7 +170,7 @@ exports.nodes = {
 
 exports.nodes.ObjectProperty =
 exports.nodes.ObjectMethod =
-exports.nodes.SpreadProperty = function (node, parent) {
+exports.nodes.SpreadProperty = function (node: Object, parent): ?WhitespaceObject {
   if (parent.properties[0] === node) {
     return {
       before: true
@@ -181,7 +188,7 @@ exports.list = {
    * Return VariableDeclaration declarations init properties.
    */
 
-  VariableDeclaration(node) {
+  VariableDeclaration(node: Object): Array<Object> {
     return map(node.declarations, "init");
   },
 
@@ -189,7 +196,7 @@ exports.list = {
    * Return VariableDeclaration elements.
    */
 
-  ArrayExpression(node) {
+  ArrayExpression(node: Object): Array<Object> {
     return node.elements;
   },
 
@@ -197,7 +204,7 @@ exports.list = {
    * Return VariableDeclaration properties.
    */
 
-  ObjectExpression(node) {
+  ObjectExpression(node: Object): Array<Object> {
     return node.properties;
   }
 };
diff --git a/packages/babel-traverse/src/path/lib/virtual-types.js b/packages/babel-traverse/src/path/lib/virtual-types.js
index e879e66577..cb598fe9a3 100644
--- a/packages/babel-traverse/src/path/lib/virtual-types.js
+++ b/packages/babel-traverse/src/path/lib/virtual-types.js
@@ -28,14 +28,14 @@ export let ReferencedMemberExpression = {
 
 export let BindingIdentifier = {
   types: ["Identifier"],
-  checkPath({ node, parent } /*: NodePath */): boolean {
+  checkPath({ node, parent }: NodePath): boolean {
     return t.isIdentifier(node) && t.isBinding(node, parent);
   }
 };
 
 export let Statement = {
   types: ["Statement"],
-  checkPath({ node, parent } /*: NodePath */): boolean {
+  checkPath({ node, parent }: NodePath): boolean {
     if (t.isStatement(node)) {
       if (t.isVariableDeclaration(node)) {
         if (t.isForXStatement(parent, { left: node })) return false;
@@ -106,7 +106,7 @@ export let Pure = {
 
 export let Flow = {
   types: ["Flow", "ImportDeclaration", "ExportDeclaration"],
-  checkPath({ node } /*: NodePath */): boolean {
+  checkPath({ node }: NodePath): boolean {
     if (t.isFlow(node)) {
       return true;
     } else if (t.isImportDeclaration(node)) {
diff --git a/packages/babel-types/src/converters.js b/packages/babel-types/src/converters.js
index 8716a10a3f..3543636a9e 100644
--- a/packages/babel-types/src/converters.js
+++ b/packages/babel-types/src/converters.js
@@ -1,3 +1,5 @@
+/* @noflow */
+
 import isPlainObject from "lodash/lang/isPlainObject";
 import isNumber from "lodash/lang/isNumber";
 import isRegExp from "lodash/lang/isRegExp";
@@ -218,7 +220,7 @@ export function toExpression(node: Object): Object {
   }
 }
 
-export function toBlock(node, parent: Object): Object {
+export function toBlock(node: Object, parent: Object): Object {
   if (t.isBlockStatement(node)) {
     return node;
   }
diff --git a/packages/babel-types/src/flow.js b/packages/babel-types/src/flow.js
index 444ea50b86..531471f2d8 100644
--- a/packages/babel-types/src/flow.js
+++ b/packages/babel-types/src/flow.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 import * as t from "./index";
 
 /**
@@ -19,7 +21,7 @@ export function createUnionTypeAnnotation(types: Array<Object>) {
  * Dedupe type annotations.
  */
 
-export function removeTypeDuplicates(nodes: Array<Object>) {
+export function removeTypeDuplicates(nodes: Array<Object>): Array<Object> {
   let generics = {};
   let bases = {};
 
diff --git a/packages/babel-types/src/index.js b/packages/babel-types/src/index.js
index 3046db78db..f3a77a26e3 100644
--- a/packages/babel-types/src/index.js
+++ b/packages/babel-types/src/index.js
@@ -1,4 +1,4 @@
-/* @noflow */
+/* @flow */
 
 import toFastProperties from "to-fast-properties";
 import compact from "lodash/array/compact";
@@ -406,6 +406,7 @@ export function inherits(child: Object, parent: Object): Object {
 
 export function assertNode(node?) {
   if (!isNode(node)) {
+    // $FlowFixMe
     throw new TypeError("Not a valid node " + (node && node.type));
   }
 }
diff --git a/packages/babel-types/src/validators.js b/packages/babel-types/src/validators.js
index e95ef2bd9a..cee738b7ab 100644
--- a/packages/babel-types/src/validators.js
+++ b/packages/babel-types/src/validators.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 import { getBindingIdentifiers } from "./retrievers";
 import esutils from "esutils";
 import * as t from "./index";
@@ -67,7 +69,7 @@ export function isReferenced(node: Object, parent: Object): boolean {
     case "ArrowFunctionExpression":
     case "FunctionDeclaration":
     case "FunctionExpression":
-      for (let param of (parent.params: Array)) {
+      for (let param of (parent.params: Array<any>)) {
         if (param === node) return false;
       }
 
diff --git a/packages/babylon/src/parser/comments.js b/packages/babylon/src/parser/comments.js
index 344ba7e2c0..38f2081322 100644
--- a/packages/babylon/src/parser/comments.js
+++ b/packages/babylon/src/parser/comments.js
@@ -118,7 +118,7 @@ pp.processComment = function (node) {
       // result in an empty array, and if so, the array must be
       // deleted.
       node.leadingComments = this.state.leadingComments.slice(0, i);
-      if (node.leadingComments.length === 0) {
+      if ((node.leadingComments: Array<any>).length === 0) {
         node.leadingComments = null;
       }
 
diff --git a/packages/babylon/src/parser/index.js b/packages/babylon/src/parser/index.js
index 1440ea1a41..14a3ff4b72 100644
--- a/packages/babylon/src/parser/index.js
+++ b/packages/babylon/src/parser/index.js
@@ -1,4 +1,4 @@
-/* @noflow */
+/* @flow */
 
 import { reservedWords } from "../util/identifier";
 import { getOptions } from "../options";
@@ -7,7 +7,7 @@ import Tokenizer from "../tokenizer";
 export const plugins = {};
 
 export default class Parser extends Tokenizer {
-  constructor(options, input: string) {
+  constructor(options: Object, input: string) {
     options = getOptions(options);
     super(options, input);
 
@@ -31,7 +31,7 @@ export default class Parser extends Tokenizer {
     this[name] = f(this[name]);
   }
 
-  loadPlugins(plugins: Array<string>) {
+  loadPlugins(plugins: Array<string>): Object {
     let pluginMap = {};
 
     if (plugins.indexOf("flow") >= 0) {
diff --git a/packages/babylon/src/tokenizer/context.js b/packages/babylon/src/tokenizer/context.js
index a8e592da6e..fb1bcc57f6 100644
--- a/packages/babylon/src/tokenizer/context.js
+++ b/packages/babylon/src/tokenizer/context.js
@@ -26,7 +26,9 @@ export class TokContext {
   override: ?Function;
 }
 
-export const types = {
+export const types: {
+  [key: string]: TokContext;
+} = {
   b_stat: new TokContext("{", false),
   b_expr: new TokContext("{", true),
   b_tmpl: new TokContext("${", true),
@@ -57,7 +59,7 @@ tt.parenR.updateContext = tt.braceR.updateContext = function () {
 
 tt.name.updateContext = function (prevType) {
   this.state.exprAllowed = false;
-  
+
   if (prevType === tt._let || prevType === tt._const || prevType === tt._var) {
     if (lineBreak.test(this.input.slice(this.state.end))) {
       this.state.exprAllowed = true;
diff --git a/packages/babylon/src/tokenizer/index.js b/packages/babylon/src/tokenizer/index.js
index 8e610f77dd..7203e30aac 100644
--- a/packages/babylon/src/tokenizer/index.js
+++ b/packages/babylon/src/tokenizer/index.js
@@ -1,3 +1,5 @@
+/* @noflow */
+
 import type { TokenType } from "./types";
 import { isIdentifierStart, isIdentifierChar, isKeyword } from "../util/identifier";
 import { types as tt, keywords as keywordTypes } from "./types";