diff --git a/.flowconfig b/.flowconfig
new file mode 100644
index 000000000..57c6722d5
--- /dev/null
+++ b/.flowconfig
@@ -0,0 +1,9 @@
+[ignore]
+.*/node_modules/documentation/*
+
+[libs]
+
+[include]
+
+[options]
+suppress_comment= \\(.\\|\n\\)*\\@FlowIgnore
diff --git a/cmds/build.js b/cmds/build.js
index 0dd4685ef..05d31839d 100644
--- a/cmds/build.js
+++ b/cmds/build.js
@@ -1,20 +1,30 @@
 'use strict'
 
+const path = require('path')
+
+const chokidar = require('chokidar')
+
 const build = require('../src/build')
 const onError = require('../src/error-handler')
+const utils = require('../src/utils')
 
 module.exports = {
   command: 'build',
   desc: 'Build ready to release',
   builder: {
-    browser: {
-      alias: 'b',
-      describe: 'Build for browser usage',
+    dist: {
+      alias: 'd',
+      describe: 'Build dist package',
+      default: true
+    },
+    lib: {
+      alias: 'l',
+      describe: 'Transpile src to lib',
       default: true
     },
-    node: {
-      alias: 'n',
-      describe: 'Build for node usage',
+    watch: {
+      alias: 'w',
+      describe: 'Keep watching source files for changes',
       default: false
     }
   },
diff --git a/package.json b/package.json
index b95b98d6f..33f23410a 100644
--- a/package.json
+++ b/package.json
@@ -30,8 +30,11 @@
   "license": "MIT",
   "dependencies": {
     "async": "^2.6.0",
+    "babel-loader": "^7.1.4",
+    "babel-preset-flow-node": "^2.0.1",
     "browserify-zlib": "^0.2.0",
     "chalk": "^2.3.0",
+    "chokidar": "^2.0.3",
     "clean-documentation-theme": "^0.5.2",
     "codecov": "^3.0.0",
     "conventional-changelog": "^1.1.7",
@@ -41,10 +44,11 @@
     "documentation": "^5.3.3",
     "es6-promisify": "^5.0.0",
     "eslint": "^4.13.0",
-    "eslint-config-aegir": "^1.0.1",
+    "eslint-config-aegir": "git+https://github.com/ipfs/eslint-config-aegir#flow",
     "execa": "^0.8.0",
     "filesize": "^3.5.11",
     "findup-sync": "^2.0.0",
+    "flow-bin": "^0.71.0",
     "fs-extra": "^4.0.3",
     "gh-pages": "^1.1.0",
     "glob": "^7.1.2",
@@ -75,6 +79,7 @@
     "safe-buffer": "^5.1.1",
     "semver": "^5.4.1",
     "simple-git": "^1.85.0",
+    "source-map-support": "^0.5.4",
     "stream-http": "^2.7.2",
     "through": "^2.3.8",
     "transform-loader": "^0.2.4",
diff --git a/src/build/browser.js b/src/build/dist.js
similarity index 75%
rename from src/build/browser.js
rename to src/build/dist.js
index 118ad738f..e891badd3 100644
--- a/src/build/browser.js
+++ b/src/build/dist.js
@@ -3,13 +3,10 @@
 const webpack = require('webpack')
 const Uglify = require('uglify-es')
 const path = require('path')
-const Listr = require('listr')
 const fs = require('fs-extra')
 const filesize = require('filesize')
 const pify = require('pify')
 
-const clean = require('../clean')
-const utils = require('../utils')
 const config = require('../config/webpack')
 
 function webpackBuild (ctx, task) {
@@ -59,19 +56,8 @@ function minify (ctx, task) {
     })
 }
 
-const TASKS = new Listr([{
-  title: 'Clean ./dist',
-  task: () => clean('dist')
-}, {
-  title: 'Webpack Build',
-  task: webpackBuild
-}, {
-  title: 'Write stats to disk',
-  task: writeStats,
-  enabled: (ctx) => ctx.webpackResult != null && ctx.stats
-}, {
-  title: 'Minify',
-  task: minify
-}], utils.getListrConfig())
-
-module.exports = TASKS
+module.exports = {
+  webpackBuild: webpackBuild,
+  writeStats: writeStats,
+  minify: minify
+}
diff --git a/src/build/index.js b/src/build/index.js
index 156696467..c0f1411f3 100644
--- a/src/build/index.js
+++ b/src/build/index.js
@@ -1,5 +1,32 @@
 'use strict'
 
-const browserTasks = require('./browser')
+const Listr = require('listr')
 
-module.exports = browserTasks
+const clean = require('../clean')
+const dist = require('./dist')
+const lib = require('./lib')
+const utils = require('../utils')
+
+const TASKS = new Listr([{
+  title: 'Clean ./dist',
+  task: () => clean('dist'),
+  enabled: (ctx) => ctx.dist
+}, {
+  title: 'Webpack Build',
+  task: dist.webpackBuild,
+  enabled: (ctx) => ctx.dist
+}, {
+  title: 'Write stats to disk',
+  task: dist.writeStats,
+  enabled: (ctx) => ctx.dist && ctx.webpackResult != null && ctx.stats
+}, {
+  title: 'Minify',
+  task: dist.minify,
+  enabled: (ctx) => ctx.dist
+}, {
+  title: 'Transpile src to lib',
+  task: lib,
+  enabled: (ctx) => ctx.lib
+}], utils.getListrConfig())
+
+module.exports = TASKS
diff --git a/src/build/lib.js b/src/build/lib.js
new file mode 100644
index 000000000..b71ad169c
--- /dev/null
+++ b/src/build/lib.js
@@ -0,0 +1,109 @@
+// Babel transform all *.js files from `src` to `lib`
+'use strict'
+
+const path = require('path')
+
+const babelCore = require('babel-core')
+const chokidar = require('chokidar')
+const fs = require('fs-extra')
+
+const utils = require('../utils')
+
+const babelConfig = {
+  env: {
+    development: {
+      sourceMaps: 'inline',
+      comments: false,
+      presets: [
+        [
+          'env',
+          {
+            targets: {
+              node: 'current'
+            }
+          }
+        ],
+        'flow-node'
+      ]
+    }
+  }
+}
+
+/**
+ * Babel transpiles a file from `src` to `lib`
+ *
+ * @param {string} filename The filename relative to the `src` directory
+ *
+ * @returns {Promise}
+ */
+const babel = (filename) => {
+  const src = path.join('src', filename)
+  const dest = path.join('lib', filename)
+
+  // To have the right filename in the source map
+  babelConfig.sourceFileName = path.join('..', src)
+
+  return new Promise((resolve, reject) => {
+    babelCore.transformFile(
+      src, babelConfig, (err, result) => {
+        if (err) {
+          return reject(err)
+        }
+        resolve(result.code)
+      })
+  }).then((code) => {
+    return fs.outputFile(dest, code)
+  })
+}
+
+/**
+ * Copies a file from `src` to `lib` with flow extension
+ *
+ * @param {string} filename The filename relative to the `src` directory
+ *
+ * @returns {Promise}
+ */
+const flowCopy = (filename) => {
+  const src = path.join('src', filename)
+  const dest = path.join('lib', filename + '.flow')
+
+  return fs.copy(src, dest)
+}
+
+const transform = (ctx) => {
+  const srcDir = path.join(utils.getBasePath(), 'src')
+
+  return new Promise((resolve, reject) => {
+    // The watcher code is based on the babel-cli code (MIT licensed):
+    // https://github.com/babel/babel/blob/6597a472b30419493f123bff1e9194e4c09e488e/packages/babel-cli/src/babel/dir.js#L164-L188`
+    const watcher = chokidar.watch(srcDir, {
+      persistent: ctx.watch,
+      ignoreInitial: false,
+      awaitWriteFinish: {
+        stabilityThreshold: 50,
+        pollInterval: 10
+      }
+    })
+
+    ;['add', 'change'].forEach((type) => {
+      watcher.on(type, (filename) => {
+        const relative = path.relative(srcDir, filename)
+        console.log('Transform file: ' + relative)
+        babel(relative)
+        flowCopy(relative)
+      })
+    })
+
+    watcher
+      .on('ready', () => {
+        // Finish the task after the initial scan. If files are watched, the
+        // task will keep running though.
+        resolve()
+      })
+      .on('error', (err) => {
+        reject(err)
+      })
+  })
+}
+
+module.exports = transform
diff --git a/src/config/webpack/index.js b/src/config/webpack/index.js
index 1dc027af3..c4250b6bd 100644
--- a/src/config/webpack/index.js
+++ b/src/config/webpack/index.js
@@ -32,6 +32,13 @@ function webpackConfig (env) {
         entry
       ],
       devtool: sourcemap,
+      module: {
+        rules: [{
+          test: /\.js$/,
+          exclude: /node_modules/,
+          loader: 'babel-loader'
+        }]
+      },
       output: {
         filename: path.basename(entry),
         library: libraryName,
diff --git a/src/lint.js b/src/lint.js
index 84ec4c19a..8c35a76eb 100644
--- a/src/lint.js
+++ b/src/lint.js
@@ -1,6 +1,10 @@
 'use strict'
 
+const util = require('util')
+const execFile = util.promisify(require('child_process').execFile)
+
 const CLIEngine = require('eslint').CLIEngine
+const flow = require('flow-bin')
 const path = require('path')
 const formatter = CLIEngine.getFormatter()
 
@@ -18,7 +22,7 @@ const FILES = [
   '!**/node_modules/**'
 ]
 
-function lint (opts) {
+const eslint = (opts) => {
   return new Promise((resolve, reject) => {
     const cli = new CLIEngine({
       useEslintrc: true,
@@ -37,4 +41,20 @@ function lint (opts) {
   })
 }
 
+const typecheck = async (_opts) => {
+  try {
+    const { stdout } = await execFile(flow, ['check', '--color=always'])
+    console.log(stdout)
+  } catch (err) {
+    console.error(err.stdout)
+    console.error(err.stderr)
+    throw new Error('Type check errors')
+  }
+}
+
+const lint = async (opts) => {
+  await eslint(opts)
+  await typecheck(opts)
+}
+
 module.exports = lint
diff --git a/src/test/node.js b/src/test/node.js
index 5e7bcd543..2bf35f64c 100644
--- a/src/test/node.js
+++ b/src/test/node.js
@@ -27,7 +27,8 @@ function testNode (ctx) {
 
   let args = [
     '--ui', 'bdd',
-    '--colors'
+    '--colors',
+    '--require', 'source-map-support/register'
   ]
 
   let files = [
diff --git a/test/lint.spec.js b/test/lint.spec.js
index 0198ea8cb..6f9691976 100644
--- a/test/lint.spec.js
+++ b/test/lint.spec.js
@@ -6,7 +6,7 @@ const lint = require('../src/lint')
 describe('lint', () => {
   it('passes', function () {
     // slow ci is slow, appveyor is even slower...
-    this.timeout(5000)
+    this.timeout(20000)
     return lint({
       fix: false
     })