diff --git a/package.json b/package.json
index 48f42c2..89053c7 100644
--- a/package.json
+++ b/package.json
@@ -29,6 +29,7 @@
   "license": "MIT",
   "devDependencies": {
     "airtap": "0.0.6",
+    "is-async-supported": "~1.2.0",
     "run-series": "~1.1.4",
     "tape": "~4.9.0"
   },
diff --git a/test/browser/callbackify.js b/test/browser/callbackify.js
new file mode 100644
index 0000000..ea05089
--- /dev/null
+++ b/test/browser/callbackify.js
@@ -0,0 +1,169 @@
+'use strict';
+
+var test = require('tape');
+var callbackify = require('../../').callbackify;
+
+if (typeof Promise === 'undefined') {
+  console.log('no global Promise found, skipping callbackify tests');
+  return;
+}
+
+function after (n, cb) {
+  var i = 0;
+  return function () {
+    if (++i === n) cb();
+  }
+}
+
+var values = [
+  'hello world',
+  null,
+  undefined,
+  false,
+  0,
+  {},
+  { key: 'value' },
+  function ok() {},
+  ['array', 'with', 4, 'values'],
+  new Error('boo')
+];
+if (typeof Symbol !== 'undefined') {
+  values.push(Symbol('I am a symbol'));
+}
+
+test('util.callbackify resolution value is passed as second argument to callback', function (t) {
+  var end = after(values.length * 2, t.end);
+  // Test that the resolution value is passed as second argument to callback
+  values.forEach(function(value) {
+    // Test Promise factory
+    function promiseFn() {
+      return Promise.resolve(value);
+    }
+
+    var cbPromiseFn = callbackify(promiseFn);
+    cbPromiseFn(function(err, ret) {
+      t.ifError(err);
+      t.strictEqual(ret, value, 'cb ' + typeof value);
+      end();
+    });
+
+    // Test Thenable
+    function thenableFn() {
+      return {
+        then: function(onRes, onRej) {
+          onRes(value);
+        }
+      };
+    }
+
+    var cbThenableFn = callbackify(thenableFn);
+    cbThenableFn(function(err, ret) {
+      t.ifError(err);
+      t.strictEqual(ret, value, 'thenable ' + typeof value);
+      end();
+    });
+  });
+});
+
+test('util.callbackify rejection reason is passed as first argument to callback', function (t) {
+  var end = after(values.length * 2, t.end);
+  // Test that rejection reason is passed as first argument to callback
+  values.forEach(function(value) {
+    // test a Promise factory
+    function promiseFn() {
+      return Promise.reject(value);
+    }
+
+    var cbPromiseFn = callbackify(promiseFn);
+    cbPromiseFn(function(err, ret) {
+      t.strictEqual(ret, undefined, 'cb ' + typeof value);
+      if (err instanceof Error) {
+        if ('reason' in err) {
+          t.ok(!value);
+          t.strictEqual(err.message, 'Promise was rejected with a falsy value');
+          t.strictEqual(err.reason, value);
+        } else {
+          t.strictEqual(String(value).slice(-err.message.length), err.message);
+        }
+      } else {
+        t.strictEqual(err, value);
+      }
+      end();
+    });
+
+    // Test Thenable
+    function thenableFn() {
+      return {
+        then: function (onRes, onRej) {
+          onRej(value);
+        }
+      };
+    }
+
+    var cbThenableFn = callbackify(thenableFn);
+    cbThenableFn(function(err, ret) {
+      t.strictEqual(ret, undefined, 'thenable ' + typeof value);
+      if (err instanceof Error) {
+        if ('reason' in err) {
+          t.ok(!value);
+          t.strictEqual(err.message, 'Promise was rejected with a falsy value');
+          t.strictEqual(err.reason, value);
+        } else {
+          t.strictEqual(String(value).slice(-err.message.length), err.message);
+        }
+      } else {
+        t.strictEqual(err, value);
+      }
+      end();
+    });
+  });
+});
+
+test('util.callbackify arguments passed to callbackified function are passed to original', function (t) {
+  var end = after(values.length, t.end);
+  // Test that arguments passed to callbackified function are passed to original
+  values.forEach(function(value) {
+    function promiseFn(arg) {
+      t.strictEqual(arg, value);
+      return Promise.resolve(arg);
+    }
+
+    var cbPromiseFn = callbackify(promiseFn);
+    cbPromiseFn(value, function(err, ret) {
+      t.ifError(err);
+      t.strictEqual(ret, value);
+      end();
+    });
+  });
+});
+
+test('util.callbackify `this` binding is the same for callbackified and original', function (t) {
+  var end = after(values.length, t.end);
+  // Test that `this` binding is the same for callbackified and original
+  values.forEach(function(value) {
+    var iAmThis = {
+      fn: function(arg) {
+        t.strictEqual(this, iAmThis);
+        return Promise.resolve(arg);
+      },
+    };
+    iAmThis.cbFn = callbackify(iAmThis.fn);
+    iAmThis.cbFn(value, function(err, ret) {
+      t.ifError(err);
+      t.strictEqual(ret, value);
+      t.strictEqual(this, iAmThis);
+      end();
+    });
+  });
+});
+
+test('util.callbackify non-function inputs throw', function (t) {
+  // Verify that non-function inputs throw.
+  ['foo', null, undefined, false, 0, {}, typeof Symbol !== 'undefined' ? Symbol() : undefined, []].forEach(function(value) {
+    t.throws(
+      function() { callbackify(value); },
+      'The "original" argument must be of type Function'
+    );
+  });
+  t.end();
+});
diff --git a/test/browser/index.js b/test/browser/index.js
index 78e0fb3..28b2e5a 100644
--- a/test/browser/index.js
+++ b/test/browser/index.js
@@ -1,2 +1,4 @@
 require('./inspect');
 require('./is');
+require('./promisify');
+require('./callbackify');
diff --git a/test/browser/promisify.js b/test/browser/promisify.js
new file mode 100644
index 0000000..44165c1
--- /dev/null
+++ b/test/browser/promisify.js
@@ -0,0 +1,205 @@
+var promisify = require('../../').promisify;
+var test = require('tape');
+
+var hasSymbol = typeof Symbol !== 'undefined';
+
+if (typeof Promise === 'undefined') {
+  console.log('no global Promise found, skipping promisify tests');
+  return;
+}
+
+var callbacker = function (arg, cb) {
+  setTimeout(function () {
+    if (typeof arg === 'string')
+      cb(null, arg.toUpperCase());
+    else cb(new TypeError('incorrect type'));
+  }, 5);
+}
+var promiser = promisify(callbacker);
+
+test('util.promisify resolves', function (t) {
+  var promise = promiser(__filename);
+  t.ok(promise instanceof Promise);
+  promise.then(function (value) {
+    t.deepEqual(value, __filename.toUpperCase());
+    t.end();
+  });
+});
+
+test('util.promisify rejects', function (t) {
+  var promise = promiser(42);
+  promise.catch(function (error) {
+    t.equal(error.message, 'incorrect type');
+    t.end();
+  });
+});
+
+test('util.promisify custom', { skip: !hasSymbol }, function (t) {
+  function fn() {}
+  function promisifedFn() {}
+  fn[promisify.custom] = promisifedFn;
+  t.strictEqual(promisify(fn), promisifedFn);
+  t.strictEqual(promisify(promisify(fn)), promisifedFn);
+  t.end();
+});
+
+test('util.promisify custom of invalid type', { skip: !hasSymbol }, function (t) {
+  function fn2() {}
+  fn2[promisify.custom] = 42;
+  t.throws(
+    function () { promisify(fn2); },
+    /must be of type Function/
+  );
+  t.end();
+});
+
+test('util.promisify multiple callback values', function (t) {
+  function fn5(callback) {
+    callback(null, 'foo', 'bar');
+  }
+  promisify(fn5)().then(function (value) {
+    t.strictEqual(value, 'foo');
+    t.end();
+  });
+});
+
+test('util.promisify no callback success value', function (t) {
+  function fn6(callback) {
+    callback(null);
+  }
+  promisify(fn6)().then(function (value) {
+    t.strictEqual(value, undefined);
+    t.end();
+  });
+});
+
+test('util.promisify no callback arguments at all', function (t) {
+  function fn7(callback) {
+    callback();
+  }
+  promisify(fn7)().then(function (value) {
+    t.strictEqual(value, undefined);
+    t.end();
+  });
+});
+
+test('util.promisify passing arguments', function (t) {
+  function fn8(err, val, callback) {
+    callback(err, val);
+  }
+  promisify(fn8)(null, 42).then(function (value) {
+    t.strictEqual(value, 42);
+    t.end();
+  });
+});
+
+test('util.promisify passing arguments (rejects)', function (t) {
+  function fn9(err, val, callback) {
+    callback(err, val);
+  }
+  promisify(fn9)(new Error('oops'), null).catch(function (err) {
+    t.strictEqual(err.message, 'oops');
+    t.end();
+  });
+});
+
+test('util.promisify chain', function (t) {
+  function fn9(err, val, callback) {
+    callback(err, val);
+  }
+
+
+  Promise.resolve()
+    .then(function () { return promisify(fn9)(null, 42); })
+    .then(function (value) {
+      t.strictEqual(value, 42);
+      t.end();
+    });
+});
+
+test('util.promisify keeps correct `this`', function (t) {
+  var o = {};
+  var fn10 = promisify(function(cb) {
+
+    cb(null, this === o);
+  });
+
+  o.fn = fn10;
+
+  o.fn().then(function(val) {
+    t.ok(val);
+    t.end();
+  });
+});
+
+test('util.promisify calling callback multiple times', function (t) {
+  var err = new Error('Should not have called the callback with the error.');
+  var stack = err.stack;
+
+  var fn11 = promisify(function(cb) {
+    cb(null);
+    cb(err);
+  });
+
+  Promise.resolve()
+    .then(function () { return fn11(); })
+    .then(function () { return Promise.resolve(); })
+    .then(function () {
+      t.strictEqual(stack, err.stack);
+      t.end();
+    });
+});
+
+// Can't do this without Symbol() unfortunately
+test('util.promisify promisifying a promisified function', { skip: !hasSymbol }, function (t) {
+  function c() { }
+  var a = promisify(function() { });
+  var b = promisify(a);
+  t.notStrictEqual(c, a);
+  t.strictEqual(a, b);
+  t.end();
+});
+
+test('util.promisify sync throw becomes rejection', function (t) {
+  var errToThrow;
+  var thrower = promisify(function(a, b, c, cb) {
+    errToThrow = new Error();
+    throw errToThrow;
+  });
+  thrower(1, 2, 3)
+    .then(t.fail)
+    .then(t.fail, function (e) {
+      t.strictEqual(e, errToThrow);
+      t.end();
+    });
+});
+
+test('util.promisify callback and sync throw', function (t) {
+  var err = new Error();
+
+  var a = promisify(function (cb) { cb(err) })();
+  var b = promisify(function () { throw err; })();
+
+  Promise.all([
+    a.then(t.fail, function(e) {
+      t.strictEqual(err, e);
+    }),
+    b.then(t.fail, function(e) {
+      t.strictEqual(err, e);
+    })
+  ]).then(function () {
+    t.end();
+  });
+});
+
+test('util.promisify throws for incorrect argument types', function (t) {
+  var array = [undefined, null, true, 0, 'str', {}, []];
+  if (typeof Symbol !== 'undefined') array.push(Symbol());
+  array.forEach(function (input) {
+    t.throws(
+      function () { promisify(input); },
+      'The "original" argument must be of type Function'
+    );
+  });
+  t.end();
+});
diff --git a/test/node/callbackify-async.js b/test/node/callbackify-async.js
new file mode 100644
index 0000000..bb8f92c
--- /dev/null
+++ b/test/node/callbackify-async.js
@@ -0,0 +1,120 @@
+'use strict';
+
+// Separate test file for tests using new syntax (async/await).
+
+var common = require('./common');
+var assert = require('assert');
+var callbackify = require('../../').callbackify;
+var execFile = require('child_process').execFile;
+
+var values = [
+  'hello world',
+  null,
+  undefined,
+  false,
+  0,
+  {},
+  { key: 'value' },
+  Symbol('I am a symbol'),
+  function ok() {},
+  ['array', 'with', 4, 'values'],
+  new Error('boo')
+];
+
+{
+  // Test that the resolution value is passed as second argument to callback
+  values.forEach(function(value) {
+    // Test and `async function`
+    async function asyncFn() {
+      return value;
+    }
+
+    var cbAsyncFn = callbackify(asyncFn);
+    cbAsyncFn(common.mustCall(function(err, ret) {
+      assert.ifError(err);
+      assert.strictEqual(ret, value);
+    }));
+  });
+}
+
+{
+  // Test that rejection reason is passed as first argument to callback
+  values.forEach(function(value) {
+    // Test an `async function`
+    async function asyncFn() {
+      return Promise.reject(value);
+    }
+
+    var cbAsyncFn = callbackify(asyncFn);
+    cbAsyncFn(common.mustCall(function (err, ret) {
+      assert.strictEqual(ret, undefined);
+      if (err instanceof Error) {
+        if ('reason' in err) {
+          assert(!value);
+          assert.strictEqual(err.message, 'Promise was rejected with a falsy value');
+          assert.strictEqual(err.reason, value);
+        } else {
+          assert.strictEqual(String(value).endsWith(err.message), true);
+        }
+      } else {
+        assert.strictEqual(err, value);
+      }
+    }));
+  });
+}
+
+{
+  // Test that arguments passed to callbackified function are passed to original
+  values.forEach(function(value) {
+    async function asyncFn(arg) {
+      assert.strictEqual(arg, value);
+      return arg;
+    }
+
+    var cbAsyncFn = callbackify(asyncFn);
+    cbAsyncFn(value, common.mustCall(function(err, ret) {
+      assert.ifError(err);
+      assert.strictEqual(ret, value);
+    }));
+  });
+}
+
+{
+  // Test that `this` binding is the same for callbackified and original
+  values.forEach(function(value) {
+    var iAmThat = {
+      async fn(arg) {
+        assert.strictEqual(this, iAmThat);
+        return arg;
+      },
+    };
+    iAmThat.cbFn = callbackify(iAmThat.fn);
+    iAmThat.cbFn(value, common.mustCall(function(err, ret) {
+      assert.ifError(err);
+      assert.strictEqual(ret, value);
+      assert.strictEqual(this, iAmThat);
+    }));
+  });
+}
+
+{
+  async function asyncFn() {
+    return 42;
+  }
+
+  var cb = callbackify(asyncFn);
+  var args = [];
+
+  // Verify that the last argument to the callbackified function is a function.
+  ['foo', null, undefined, false, 0, {}, Symbol(), []].forEach(function(value) {
+    args.push(value);
+    common.expectsError(function() {
+      cb(...args);
+    }, {
+      code: 'ERR_INVALID_ARG_TYPE',
+      type: TypeError,
+      message: 'The last argument must be of type Function'
+    });
+  });
+}
+
diff --git a/test/node/callbackify.js b/test/node/callbackify.js
new file mode 100644
index 0000000..a3828ab
--- /dev/null
+++ b/test/node/callbackify.js
@@ -0,0 +1,190 @@
+'use strict';
+
+// This test checks that the semantics of `util.callbackify` are as described in
+// the API docs
+
+var common = require('./common');
+var assert = require('assert');
+var callbackify = require('../../').callbackify;
+var execFile = require('child_process').execFile;
+
+var values = [
+  'hello world',
+  null,
+  undefined,
+  false,
+  0,
+  {},
+  { key: 'value' },
+  function ok() {},
+  ['array', 'with', 4, 'values'],
+  new Error('boo')
+];
+if (typeof Symbol !== 'undefined') {
+  values.push(Symbol('I am a symbol'));
+}
+
+{
+  // Test that the resolution value is passed as second argument to callback
+  values.forEach(function(value) {
+    // Test Promise factory
+    function promiseFn() {
+      return Promise.resolve(value);
+    }
+
+    var cbPromiseFn = callbackify(promiseFn);
+    cbPromiseFn(common.mustCall(function(err, ret) {
+      assert.ifError(err);
+      assert.strictEqual(ret, value);
+    }));
+
+    // Test Thenable
+    function thenableFn() {
+      return {
+        then: function(onRes, onRej) {
+          onRes(value);
+        }
+      };
+    }
+
+    var cbThenableFn = callbackify(thenableFn);
+    cbThenableFn(common.mustCall(function(err, ret) {
+      assert.ifError(err);
+      assert.strictEqual(ret, value);
+    }));
+  });
+}
+
+{
+  // Test that rejection reason is passed as first argument to callback
+  values.forEach(function(value) {
+    // test a Promise factory
+    function promiseFn() {
+      return Promise.reject(value);
+    }
+
+    var cbPromiseFn = callbackify(promiseFn);
+    cbPromiseFn(common.mustCall(function(err, ret) {
+      assert.strictEqual(ret, undefined);
+      if (err instanceof Error) {
+        if ('reason' in err) {
+          assert(!value);
+          assert.strictEqual(err.message, 'Promise was rejected with a falsy value');
+          assert.strictEqual(err.reason, value);
+        } else {
+          assert.strictEqual(String(value).endsWith(err.message), true);
+        }
+      } else {
+        assert.strictEqual(err, value);
+      }
+    }));
+
+    // Test Thenable
+    function thenableFn() {
+      return {
+        then: function (onRes, onRej) {
+          onRej(value);
+        }
+      };
+    }
+
+    var cbThenableFn = callbackify(thenableFn);
+    cbThenableFn(common.mustCall(function(err, ret) {
+      assert.strictEqual(ret, undefined);
+      if (err instanceof Error) {
+        if ('reason' in err) {
+          assert(!value);
+          assert.strictEqual(err.message, 'Promise was rejected with a falsy value');
+          assert.strictEqual(err.reason, value);
+        } else {
+          assert.strictEqual(String(value).endsWith(err.message), true);
+        }
+      } else {
+        assert.strictEqual(err, value);
+      }
+    }));
+  });
+}
+
+{
+  // Test that arguments passed to callbackified function are passed to original
+  values.forEach(function(value) {
+    function promiseFn(arg) {
+      assert.strictEqual(arg, value);
+      return Promise.resolve(arg);
+    }
+
+    var cbPromiseFn = callbackify(promiseFn);
+    cbPromiseFn(value, common.mustCall(function(err, ret) {
+      assert.ifError(err);
+      assert.strictEqual(ret, value);
+    }));
+  });
+}
+
+{
+  // Test that `this` binding is the same for callbackified and original
+  values.forEach(function(value) {
+    var iAmThis = {
+      fn: function(arg) {
+        assert.strictEqual(this, iAmThis);
+        return Promise.resolve(arg);
+      },
+    };
+    iAmThis.cbFn = callbackify(iAmThis.fn);
+    iAmThis.cbFn(value, common.mustCall(function(err, ret) {
+      assert.ifError(err);
+      assert.strictEqual(ret, value);
+      assert.strictEqual(this, iAmThis);
+    }));
+  });
+}
+
+// These tests are not necessary in the browser.
+if (false) {
+  // Test that callback that throws emits an `uncaughtException` event
+  var fixture = fixtures.path('uncaught-exceptions', 'callbackify1.js');
+  execFile(
+    process.execPath,
+    [fixture],
+    common.mustCall(function (err, stdout, stderr) {
+      assert.strictEqual(err.code, 1);
+      assert.strictEqual(Object.getPrototypeOf(err).name, 'Error');
+      assert.strictEqual(stdout, '');
+      var errLines = stderr.trim().split(/[\r\n]+/);
+      var errLine = errLines.find(function (l) { return /^Error/.exec(l) });
+      assert.strictEqual(errLine, 'Error: ' + fixture);
+    })
+  );
+}
+
+if (false) {
+  // Test that handled `uncaughtException` works and passes rejection reason
+  var fixture = fixtures.path('uncaught-exceptions', 'callbackify2.js');
+  execFile(
+    process.execPath,
+    [fixture],
+    common.mustCall(function (err, stdout, stderr) {
+      assert.ifError(err);
+      assert.strictEqual(stdout.trim(), fixture);
+      assert.strictEqual(stderr, '');
+    })
+  );
+}
+
+{
+  // Verify that non-function inputs throw.
+  ['foo', null, undefined, false, 0, {}, typeof Symbol !== 'undefined' ? Symbol() : undefined, []].forEach(function(value) {
+    common.expectsError(function() {
+      callbackify(value);
+    }, {
+      code: 'ERR_INVALID_ARG_TYPE',
+      type: TypeError,
+      message: 'The "original" argument must be of type Function'
+    });
+  });
+}
+
+if (require('is-async-supported')()) {
+  require('./callbackify-async');
+}
diff --git a/test/node/common.js b/test/node/common.js
new file mode 100644
index 0000000..b5b9d58
--- /dev/null
+++ b/test/node/common.js
@@ -0,0 +1,31 @@
+var assert = require('assert');
+
+var mustCalls = [];
+var common = {
+  expectsError: function (fn, props) {
+    try { fn(); }
+    catch (err) {
+      if (props.type) assert.equal(err.constructor, props.type);
+      if (props.message) assert.equal(err.message, props.message);
+      return;
+    }
+    assert.fail('expected error');
+  },
+  mustCall: function (fn) {
+    function mustCall() {
+      mustCall.called = true
+      return fn.apply(this, arguments);
+    }
+
+    mustCalls.push(mustCall);
+    return mustCall;
+  }
+};
+
+process.on('exit', function () {
+  mustCalls.forEach(function (mc) {
+    assert(mc.called);
+  });
+});
+
+module.exports = common;
diff --git a/test/node/index.js b/test/node/index.js
index 6ad5b42..a3a807f 100644
--- a/test/node/index.js
+++ b/test/node/index.js
@@ -15,5 +15,7 @@ series([
   test(require.resolve('./debug')),
   test(require.resolve('./format')),
   test(require.resolve('./inspect')),
-  test(require.resolve('./log'))
+  test(require.resolve('./log')),
+  test(require.resolve('./promisify')),
+  test(require.resolve('./callbackify'))
 ]);
diff --git a/test/node/promisify.js b/test/node/promisify.js
new file mode 100644
index 0000000..57afcf0
--- /dev/null
+++ b/test/node/promisify.js
@@ -0,0 +1,201 @@
+var common = require('./common');
+var assert = require('assert');
+var fs = require('fs');
+var vm = require('vm');
+var promisify = require('../../util').promisify;
+
+if (typeof Promise === 'undefined') {
+  console.log('no global Promise found, skipping promisify tests');
+  return;
+}
+
+var stat = promisify(fs.stat);
+
+{
+  var promise = stat(__filename);
+  assert(promise instanceof Promise);
+  promise.then(common.mustCall(function (value) {
+    assert.deepStrictEqual(value, fs.statSync(__filename));
+  }));
+}
+
+{
+  var promise = stat('/dontexist');
+  promise.catch(common.mustCall(function (error) {
+    assert(error.message.indexOf('ENOENT: no such file or directory, stat') !== -1);
+  }));
+}
+
+{
+  function fn() {}
+  function promisifedFn() {}
+  fn[promisify.custom] = promisifedFn;
+  assert.strictEqual(promisify(fn), promisifedFn);
+  assert.strictEqual(promisify(promisify(fn)), promisifedFn);
+}
+
+{
+  function fn2() {}
+  fn2[promisify.custom] = 42;
+  common.expectsError(
+    function () { promisify(fn2); },
+    { code: 'ERR_INVALID_ARG_TYPE', type: TypeError }
+  );
+}
+
+// promisify args test disabled, it is an internal core API that is
+// not used anywhere anymore and this package does not implement it.
+if (false) {
+  var firstValue = 5;
+  var secondValue = 17;
+
+  function fn3(callback) {
+    callback(null, firstValue, secondValue);
+  }
+
+  fn3[customPromisifyArgs] = ['first', 'second'];
+
+  promisify(fn3)().then(common.mustCall(function (obj) {
+    assert.deepStrictEqual(obj, { first: firstValue, second: secondValue });
+  }));
+}
+
+{
+  var fn4 = vm.runInNewContext('(function() {})');
+  assert.notStrictEqual(Object.getPrototypeOf(promisify(fn4)),
+                        Function.prototype);
+}
+
+{
+  function fn5(callback) {
+    callback(null, 'foo', 'bar');
+  }
+  promisify(fn5)().then(common.mustCall(function (value) {
+    assert.deepStrictEqual(value, 'foo');
+  }));
+}
+
+{
+  function fn6(callback) {
+    callback(null);
+  }
+  promisify(fn6)().then(common.mustCall(function (value) {
+    assert.strictEqual(value, undefined);
+  }));
+}
+
+{
+  function fn7(callback) {
+    callback();
+  }
+  promisify(fn7)().then(common.mustCall(function (value) {
+    assert.strictEqual(value, undefined);
+  }));
+}
+
+{
+  function fn8(err, val, callback) {
+    callback(err, val);
+  }
+  promisify(fn8)(null, 42).then(common.mustCall(function (value) {
+    assert.strictEqual(value, 42);
+  }));
+}
+
+{
+  function fn9(err, val, callback) {
+    callback(err, val);
+  }
+  promisify(fn9)(new Error('oops'), null).catch(common.mustCall(function (err) {
+    assert.strictEqual(err.message, 'oops');
+  }));
+}
+
+{
+  function fn9(err, val, callback) {
+    callback(err, val);
+  }
+
+
+  Promise.resolve()
+    .then(function () { return promisify(fn9)(null, 42); })
+    .then(function (value) {
+      assert.strictEqual(value, 42);
+    });
+}
+
+{
+  var o = {};
+  var fn10 = promisify(function(cb) {
+
+    cb(null, this === o);
+  });
+
+  o.fn = fn10;
+
+  o.fn().then(common.mustCall(function(val) {
+    assert(val);
+  }));
+}
+
+(function () {
+  var err = new Error('Should not have called the callback with the error.');
+  var stack = err.stack;
+
+  var fn11 = promisify(function(cb) {
+    cb(null);
+    cb(err);
+  });
+
+  Promise.resolve()
+    .then(function () { return fn11(); })
+    .then(function () { return Promise.resolve(); })
+    .then(function () {
+      assert.strictEqual(stack, err.stack);
+    });
+})();
+
+{
+  function c() { }
+  var a = promisify(function() { });
+  var b = promisify(a);
+  assert.notStrictEqual(c, a);
+  assert.strictEqual(a, b);
+}
+
+{
+  var errToThrow;
+  var thrower = promisify(function(a, b, c, cb) {
+    errToThrow = new Error();
+    throw errToThrow;
+  });
+  thrower(1, 2, 3)
+    .then(assert.fail)
+    .then(assert.fail, function (e) { assert.strictEqual(e, errToThrow); });
+}
+
+{
+  var err = new Error();
+
+  var a = promisify(function (cb) { cb(err) })();
+  var b = promisify(function () { throw err; })();
+
+  Promise.all([
+    a.then(assert.fail, function(e) {
+      assert.strictEqual(err, e);
+    }),
+    b.then(assert.fail, function(e) {
+      assert.strictEqual(err, e);
+    })
+  ]);
+}
+
+[undefined, null, true, 0, 'str', {}, [], Symbol()].forEach(function (input) {
+  common.expectsError(
+    function () { promisify(input); },
+    {
+      code: 'ERR_INVALID_ARG_TYPE',
+      type: TypeError,
+      message: 'The "original" argument must be of type Function'
+    });
+});
diff --git a/util.js b/util.js
index e0ea321..cbc2498 100644
--- a/util.js
+++ b/util.js
@@ -19,6 +19,16 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
+var getOwnPropertyDescriptors = Object.getOwnPropertyDescriptors ||
+  function getOwnPropertyDescriptors(obj) {
+    var keys = Object.keys(obj);
+    var descriptors = {};
+    for (var i = 0; i < keys.length; i++) {
+      descriptors[keys[i]] = Object.getOwnPropertyDescriptor(obj, keys[i]);
+    }
+    return descriptors;
+  };
+
 var formatRegExp = /%[sdj%]/g;
 exports.format = function(f) {
   if (!isString(f)) {
@@ -584,3 +594,110 @@ exports._extend = function(origin, add) {
 function hasOwnProperty(obj, prop) {
   return Object.prototype.hasOwnProperty.call(obj, prop);
 }
+
+var kCustomPromisifiedSymbol = typeof Symbol !== 'undefined' ? Symbol('util.promisify.custom') : undefined;
+
+exports.promisify = function promisify(original) {
+  if (typeof original !== 'function')
+    throw new TypeError('The "original" argument must be of type Function');
+
+  if (kCustomPromisifiedSymbol && original[kCustomPromisifiedSymbol]) {
+    var fn = original[kCustomPromisifiedSymbol];
+    if (typeof fn !== 'function') {
+      throw new TypeError('The "util.promisify.custom" argument must be of type Function');
+    }
+    Object.defineProperty(fn, kCustomPromisifiedSymbol, {
+      value: fn, enumerable: false, writable: false, configurable: true
+    });
+    return fn;
+  }
+
+  function fn() {
+    var promiseResolve, promiseReject;
+    var promise = new Promise(function (resolve, reject) {
+      promiseResolve = resolve;
+      promiseReject = reject;
+    });
+
+    var args = [];
+    for (var i = 0; i < arguments.length; i++) {
+      args.push(arguments[i]);
+    }
+    args.push(function (err, value) {
+      if (err) {
+        promiseReject(err);
+      } else {
+        promiseResolve(value);
+      }
+    });
+
+    try {
+      original.apply(this, args);
+    } catch (err) {
+      promiseReject(err);
+    }
+
+    return promise;
+  }
+
+  Object.setPrototypeOf(fn, Object.getPrototypeOf(original));
+
+  if (kCustomPromisifiedSymbol) Object.defineProperty(fn, kCustomPromisifiedSymbol, {
+    value: fn, enumerable: false, writable: false, configurable: true
+  });
+  return Object.defineProperties(
+    fn,
+    getOwnPropertyDescriptors(original)
+  );
+}
+
+exports.promisify.custom = kCustomPromisifiedSymbol
+
+function callbackifyOnRejected(reason, cb) {
+  // `!reason` guard inspired by bluebird (Ref: https://goo.gl/t5IS6M).
+  // Because `null` is a special error value in callbacks which means "no error
+  // occurred", we error-wrap so the callback consumer can distinguish between
+  // "the promise rejected with null" or "the promise fulfilled with undefined".
+  if (!reason) {
+    var newReason = new Error('Promise was rejected with a falsy value');
+    newReason.reason = reason;
+    reason = newReason;
+  }
+  return cb(reason);
+}
+
+function callbackify(original) {
+  if (typeof original !== 'function') {
+    throw new TypeError('The "original" argument must be of type Function');
+  }
+
+  // We DO NOT return the promise as it gives the user a false sense that
+  // the promise is actually somehow related to the callback's execution
+  // and that the callback throwing will reject the promise.
+  function callbackified() {
+    var args = [];
+    for (var i = 0; i < arguments.length; i++) {
+      args.push(arguments[i]);
+    }
+
+    var maybeCb = args.pop();
+    if (typeof maybeCb !== 'function') {
+      throw new TypeError('The last argument must be of type Function');
+    }
+    var self = this;
+    var cb = function() {
+      return maybeCb.apply(self, arguments);
+    };
+    // In true node style we process the callback on `nextTick` with all the
+    // implications (stack, `uncaughtException`, `async_hooks`)
+    original.apply(this, args)
+      .then(function(ret) { process.nextTick(cb, null, ret) },
+            function(rej) { process.nextTick(callbackifyOnRejected, rej, cb) });
+  }
+
+  Object.setPrototypeOf(callbackified, Object.getPrototypeOf(original));
+  Object.defineProperties(callbackified,
+                          getOwnPropertyDescriptors(original));
+  return callbackified;
+}
+exports.callbackify = callbackify;