diff --git a/.eslintrc b/.eslintrc
index 81d1af35..726bf23d 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -1,5 +1,10 @@
 {
   "root": true,
   "parser": "babel-eslint",
-  "extends": "vue"
+  "extends": "vue",
+  "plugins": ["flow-vars"],
+  "rules": {
+    "flow-vars/define-flow-type": 1,
+    "flow-vars/use-flow-type": 1
+  }
 }
diff --git a/package.json b/package.json
index 59005241..87943c60 100644
--- a/package.json
+++ b/package.json
@@ -49,6 +49,7 @@
     "entities": "1.1.x",
     "eslint": "2.9.x",
     "eslint-config-vue": "1.0.x",
+    "eslint-plugin-flow-vars": "^0.4.0",
     "flow-bin": "^0.24.2",
     "http-server": "0.9.x",
     "isparta-loader": "2.0.x",
diff --git a/src/entries/web-runtime-with-compiler.js b/src/entries/web-runtime-with-compiler.js
index f963a702..3cc33dd2 100644
--- a/src/entries/web-runtime-with-compiler.js
+++ b/src/entries/web-runtime-with-compiler.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 import config from 'core/config'
 import { warn, cached } from 'core/util/index'
 import { query } from 'web/util/index'
@@ -7,7 +9,7 @@ import { compileToFunctions } from './web-compiler'
 const idToTemplate = cached(id => query(id).innerHTML)
 const mount = Vue.prototype.$mount
 
-Vue.prototype.$mount = function (el) {
+Vue.prototype.$mount = function (el?: string | Element): Vue {
   el = el && query(el)
   const options = this.$options
   // resolve template/el and convert to render function
@@ -42,11 +44,8 @@ Vue.prototype.$mount = function (el) {
 /**
  * Get outerHTML of elements, taking care
  * of SVG elements in IE as well.
- *
- * @param {Element} el
- * @return {String}
  */
-function getOuterHTML (el) {
+function getOuterHTML (el: Element): string {
   if (el.outerHTML) {
     return el.outerHTML
   } else {
diff --git a/src/entries/web-runtime.js b/src/entries/web-runtime.js
index 3cc5c503..232467b8 100644
--- a/src/entries/web-runtime.js
+++ b/src/entries/web-runtime.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 import Vue from 'core/index'
 import config from 'core/config'
 import { createPatchFunction } from 'core/vdom/patch'
@@ -22,7 +24,7 @@ Vue.prototype.__patch__ = config._isServer
   : createPatchFunction({ nodeOps, modules })
 
 // wrap mount
-Vue.prototype.$mount = function (el) {
+Vue.prototype.$mount = function (el?: string | Element): Vue {
   this.$el = el && query(el)
   return this._mount()
 }
diff --git a/src/entries/web-server-renderer.js b/src/entries/web-server-renderer.js
index 4c43f112..b369cc90 100644
--- a/src/entries/web-server-renderer.js
+++ b/src/entries/web-server-renderer.js
@@ -1,9 +1,14 @@
+/* @flow */
+
 import { createRenderer } from 'server/create-renderer'
 import { isUnaryTag } from 'web/util/index'
 import modules from 'web/server/modules/index'
 import baseDirectives from 'web/server/directives/index'
 
-export default function publicCreateRenderer (options = {}) {
+export default function publicCreateRenderer (options?: Object = {}): {
+  renderToString: Function,
+  renderToStream: Function
+} {
   // user can provide server-side implementations for custom directives
   // when creating the renderer.
   const directives = Object.assign(baseDirectives, options.directives)