diff --git a/.gitignore b/.gitignore
index 5902c15..f7ce2bc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,4 @@
 /coverage/
 /lib/
 /node_modules/
-/npm-debug.log
+/npm-debug.*
diff --git a/.npmignore b/.npmignore
index d71ab34..d80ec38 100644
--- a/.npmignore
+++ b/.npmignore
@@ -1,5 +1,6 @@
 /coverage/
 /src/
+/test/
 /.babelrc
 /.editorconfig
 /.eslintrc.yml
diff --git a/README.md b/README.md
index e4007bb..a10b108 100644
--- a/README.md
+++ b/README.md
@@ -10,13 +10,6 @@
 [Gulp][Gulp link] plugin for applying arbitrary transformations to
 the contents of files.
 
-* **Simple**. Just pass a callback function that takes the current file
-contents and returns the desired contents.
-* **Flexible**. Receive file contents as a Buffer or a string. Compatible with
-pipelines in both buffer mode and streaming mode.
-* **Economical**. Reduce the need for gulp-specific plugins by pairing
-gulp-transform with ordinary node packages and functions.
-
 ## Install
 
 Install via [npm][NPM link]:
@@ -89,9 +82,10 @@ gulp.task('cheerio', function() {
 
 ##### transformFn `function`
 
-The callback responsible for the transformation. The return value must be a
-string or a Buffer, which will replace the file's contents. The callback
-is invoked once per file with the following arguments:
+The callback responsible for the transformation, whose return value will replace
+the file's contents. The return value may be a string, a Buffer, or a Promise
+resolvable to a string or Buffer. The callback is invoked once per file with the
+following arguments:
 
 * **contents** `Buffer` | `string` <br>
 The initial contents of the file. Contents are passed as a Buffer unless the
diff --git a/package.json b/package.json
index 2ec675e..947e459 100644
--- a/package.json
+++ b/package.json
@@ -38,6 +38,7 @@
     "gulp": "3.x"
   },
   "dependencies": {
+    "es6-promise": "^4.0.5",
     "gulp-util": "^3.0.7",
     "lodash": "^4.13.1"
   },
@@ -49,10 +50,10 @@
     "chai": "^3.5.0",
     "coffee-script": "^1.10.0",
     "coveralls": "^2.11.9",
-    "eslint": "^2.11.1",
+    "eslint": "^2.13.1",
     "event-stream": "^3.3.2",
-    "istanbul": "1.0.0-alpha.2",
-    "mocha": "^2.5.3",
+    "istanbul": "1.1.0-alpha.1",
+    "mocha": "^3.1.2",
     "rimraf": "^2.5.2",
     "sinon": "^1.17.4",
     "sinon-chai": "^2.8.0"
diff --git a/src/file-stream.js b/src/file-stream.js
index d2250d6..e2da43b 100644
--- a/src/file-stream.js
+++ b/src/file-stream.js
@@ -19,9 +19,13 @@ export class FileStream extends Transform {
 
   _flush(done) {
     let contents = Buffer.concat(this.data);
-    this.push(transform(this.fn, contents, this.file, this.opts));
 
-    done();
+    transform(this.fn, contents, this.file, this.opts).then((result) => {
+      this.push(result);
+      done();
+    }).catch((err) => {
+      done(err);
+    });
   }
 
 }
diff --git a/src/index.js b/src/index.js
index c7ef43f..662fe5b 100644
--- a/src/index.js
+++ b/src/index.js
@@ -5,13 +5,10 @@ import {err} from './err';
 export default function gulpTransform(transformFn, options) {
   if (isNil(transformFn)) {
     err('transformFn must be defined');
-
   } else if (!isFunction(transformFn)) {
     err('transformFn must be a function');
-
   } else if (!isNil(options) && !isObject(options)) {
     err('options must be an object');
-
   } else {
     return new PluginStream(transformFn, options);
   }
diff --git a/src/plugin-stream.js b/src/plugin-stream.js
index 2fc649e..1de084c 100644
--- a/src/plugin-stream.js
+++ b/src/plugin-stream.js
@@ -15,14 +15,17 @@ export class PluginStream extends Transform {
     let {fn, opts} = this;
 
     if (file.isBuffer()) {
-      file.contents = transform(fn, file.contents, file, opts);
-    }
-
-    if (file.isStream()) {
+      transform(fn, file.contents, file, opts).then((result) => {
+        file.contents = result;
+        next(null, file);
+      }).catch((err) => {
+        next(err);
+      });
+    } else if (file.isStream()) {
       file.contents = file.contents.pipe(new FileStream(fn, file, opts));
+      next(null, file);
+    } else {
+      next(null, file);
     }
-
-    next(null, file);
   }
-
 }
diff --git a/src/transform.js b/src/transform.js
index fe09458..f4bad6d 100644
--- a/src/transform.js
+++ b/src/transform.js
@@ -1,11 +1,20 @@
-import {isBuffer, isString} from 'lodash';
+import {Promise} from 'es6-promise';
+import {isBuffer} from 'lodash';
 import {err} from './err';
 
-export function transform(fn, contents, file, opts) {
-  let encoded = opts.encoding ? contents.toString(opts.encoding) : contents;
-  let transformed = fn.call(opts.thisArg, encoded, file);
+export function transform(fn, contents, file, {encoding, thisArg}) {
+  let decoded = encoding ? contents.toString(encoding) : contents;
+  let transformed = fn.call(thisArg, decoded, file);
 
-  return isBuffer(transformed) ? transformed :
-    isString(transformed) ? new Buffer(transformed) :
-    err('transformFn must return a string or a Buffer');
+  return Promise.resolve(transformed).then(toBuffer);
+}
+
+function toBuffer(contents) {
+  if (isBuffer(contents)) {
+    return contents;
+  } else if (contents != null) {
+    return new Buffer(String(contents));
+  } else {
+    err('transformFn may not return or resolve to null or undefined');
+  }
 }
diff --git a/test/fixtures/fn.coffee b/test/fixtures/fn.coffee
index a9c5a6c..cf925c9 100644
--- a/test/fixtures/fn.coffee
+++ b/test/fixtures/fn.coffee
@@ -1,3 +1,4 @@
+{Promise} = require 'es6-promise';
 {spy} = require 'sinon'
 
 re = /one|two|three/g
@@ -7,11 +8,17 @@ dict =
   two:   'deux'
   three: 'trois'
 
+translate = (content) ->
+  content.replace(re, (match) -> dict[match])
+
 exports.stringFn = ->
   spy (content) ->
-    content.replace re, (match) ->
-      dict[match]
+    translate(content)
 
 exports.bufferFn = ->
   spy (content) ->
     Buffer.concat([content, content])
+
+exports.asyncFn = ->
+  spy (content) ->
+    return Promise.resolve(translate(content))
diff --git a/test/index.coffee b/test/index.coffee
index 17cb3cb..c3285c6 100644
--- a/test/index.coffee
+++ b/test/index.coffee
@@ -1,11 +1,11 @@
 chai = require 'chai'
 sinonChai = require 'sinon-chai'
 {match: {any, instanceOf}} = require 'sinon'
-{File: {isVinyl}} = require 'gulp-util'
+{File: {isVinyl}, PluginError} = require 'gulp-util'
 {wait} = require 'event-stream'
 {buffer, string} = require './fixtures/content'
 {buffered, streaming} = require './fixtures/file'
-{bufferFn, stringFn} = require './fixtures/fn'
+{bufferFn, stringFn, asyncFn} = require './fixtures/fn'
 err = require './helpers/err'
 transform = require '../src'
 
@@ -30,10 +30,14 @@ describe 'plugin: gulp-transform', ->
       it 'throws PluginError', ->
         err -> transform 42
 
-    context 'returns neither a string nor a Buffer', ->
+    context 'returns null or undefined', ->
 
-      it 'throws PluginError', ->
-        err -> transform((content) -> 42).write buffered()
+      it 'emits PluginError', (done) ->
+        stream = transform((content) -> null)
+        stream.write buffered()
+        stream.on 'error', (err) ->
+          err.should.be.instanceOf PluginError
+          done()
 
     context 'returns a Buffer or string', ->
       [fn, file] = [null, null]
@@ -52,6 +56,23 @@ describe 'plugin: gulp-transform', ->
       it 'is called with vinyl File as second argument', ->
         fn.should.have.been.calledWith any, file
 
+    context 'returns a Promise that resolves to a string or Buffer', ->
+      [fn, file] = [null, null]
+
+      beforeEach ->
+        file = buffered()
+        fn = asyncFn()
+        transform(fn, {encoding: 'utf8'}).write(file)
+
+      it 'is called once per file', ->
+        fn.should.have.been.calledOnce
+
+      it 'is called with contents as first argument', ->
+        fn.should.have.been.calledWith string
+
+      it 'is called with vinyl File as second argument', ->
+        fn.should.have.been.calledWith any, file
+
   describe 'param: options', ->
 
     context 'not an object', ->
@@ -80,40 +101,83 @@ describe 'plugin: gulp-transform', ->
         fn.should.have.been.calledOn undefined
 
     describe 'mode: buffer', ->
-      file = null
 
-      beforeEach (done) ->
-        transform(bufferFn()).once('data', (_file) ->
-          file = _file
-          done()
-        ).write buffered()
+      context 'synchronous', ->
+        file = null
+
+        beforeEach (done) ->
+          transform(bufferFn()).once('data', (_file) ->
+            file = _file
+            done()
+          ).write buffered()
+
+        it 'returns a stream of vinyl Files', ->
+          isVinyl(file).should.be.true
+
+        it 'files are in buffer mode', ->
+          file.isBuffer().should.be.true;
 
-      it 'returns a stream of vinyl Files', ->
-        isVinyl(file).should.be.true
+        it 'transforms file contents', ->
+          file.contents.should.deep.equal Buffer.concat([buffer, buffer])
 
-      it 'files are in buffer mode', ->
-        file.isBuffer().should.be.true;
+      context 'async', ->
+        file = null
 
-      it 'transforms file contents', ->
-        file.contents.should.deep.equal Buffer.concat([buffer, buffer])
+        beforeEach (done) ->
+          transform(asyncFn(), {encoding: 'utf8'}).once('data', (_file) ->
+            file = _file
+            done()
+          ).write buffered()
+
+        it 'returns a stream of vinyl Files', ->
+          isVinyl(file).should.be.true
+
+        it 'files are in buffer mode', ->
+          file.isBuffer().should.be.true;
+
+        it 'transforms file contents', ->
+          file.contents.should.deep.equal new Buffer('un deux trois')
 
     describe 'mode: streaming', ->
-      file = null
 
-      beforeEach (done) ->
-        transform(stringFn(), {encoding: 'utf8'}).once('data', (_file) ->
-          file = _file
-          done()
-        ).write streaming()
+      context 'synchronous', ->
+        file = null
 
-      it 'returns a stream of vinyl Files', ->
-        isVinyl(file).should.be.true
+        beforeEach (done) ->
+          transform(stringFn(), {encoding: 'utf8'}).once('data', (_file) ->
+            file = _file
+            done()
+          ).write streaming()
 
-      it 'files are in streaming mode', ->
-        file.isStream().should.be.true
+        it 'returns a stream of vinyl Files', ->
+          isVinyl(file).should.be.true
 
-      it 'transforms file contents', (done) ->
-        file.pipe(wait((err, data) ->
-          data.should.deep.equal new Buffer('un deux trois')
-          done()
-        ))
+        it 'files are in streaming mode', ->
+          file.isStream().should.be.true
+
+        it 'transforms file contents', (done) ->
+          file.pipe(wait((err, data) ->
+            data.should.deep.equal new Buffer('un deux trois')
+            done()
+          ))
+
+      context 'async', ->
+        file = null
+
+        beforeEach (done) ->
+          transform(asyncFn(), {encoding: 'utf8'}).once('data', (_file) ->
+            file = _file
+            done()
+          ).write streaming()
+
+        it 'returns a stream of vinyl Files', ->
+          isVinyl(file).should.be.true
+
+        it 'files are in streaming mode', ->
+          file.isStream().should.be.true
+
+        it 'transforms file contents', (done) ->
+          file.pipe(wait((err, data) ->
+            data.should.deep.equal new Buffer('un deux trois')
+            done()
+          ))