diff --git a/benchmarks/middleware.js b/benchmarks/middleware.js
index e082cf1..d23755a 100644
--- a/benchmarks/middleware.js
+++ b/benchmarks/middleware.js
@@ -11,16 +11,17 @@ let n = parseInt(process.env.MW || '1', 10);
 console.log(`  ${n} middleware`);
 
 while (n--) {
-  app.use(function *(ctx, next){
-    yield next();
+  app.use(function(ctx, next){
+    return next();
   });
 }
 
 const body = new Buffer('Hello World');
 
-app.use(function *(ctx, next){
-  yield next();
-  this.body = body;
+app.use(function(ctx, next){
+  return next().then(function(){
+    this.body = body;
+  });
 });
 
 app.listen(3333);
diff --git a/lib/application.js b/lib/application.js
index f4a59eb..b6e2b03 100644
--- a/lib/application.js
+++ b/lib/application.js
@@ -21,7 +21,6 @@ const assert = require('assert');
 const Stream = require('stream');
 const http = require('http');
 const only = require('only');
-const co = require('co');
 
 /**
  * Expose `Application` class.
@@ -93,7 +92,9 @@ module.exports = class Application extends Emitter {
   use(fn) {
     debug('use %s', fn._name || fn.name || '-');
     if (typeof fn !== 'function') throw new TypeError('middleware must be a function!');
-    if (isGeneratorFunction(fn)) fn = co.wrap(fn);
+    if (isGeneratorFunction(fn)) {
+      throw new TypeError('Support for generators has been removed. Use Promises or wrap your generator with co.wrap');
+    }
     this.middleware.push(fn);
     return this;
   }
diff --git a/package.json b/package.json
index 78867fb..8176819 100644
--- a/package.json
+++ b/package.json
@@ -19,7 +19,6 @@
   "license": "MIT",
   "dependencies": {
     "accepts": "^1.2.2",
-    "co": "^4.4.0",
     "content-disposition": "~0.5.0",
     "content-type": "^1.0.0",
     "cookies": "~0.5.0",
diff --git a/test/application/context.js b/test/application/context.js
index 718c148..257cb57 100644
--- a/test/application/context.js
+++ b/test/application/context.js
@@ -11,7 +11,7 @@ describe('app.context', function(){
   const app2 = new Koa();
 
   it('should merge properties', function(done){
-    app1.use(function *(ctx, next){
+    app1.use(function(ctx, next){
       assert.equal(ctx.msg, 'hello');
       ctx.status = 204;
     });
@@ -22,7 +22,7 @@ describe('app.context', function(){
   });
 
   it('should not affect the original prototype', function(done){
-    app2.use(function *(ctx, next){
+    app2.use(function(ctx, next){
       assert.equal(ctx.msg, undefined);
       ctx.status = 204;
     });
diff --git a/test/application/index.js b/test/application/index.js
index 9dd7e5c..86320bb 100644
--- a/test/application/index.js
+++ b/test/application/index.js
@@ -9,7 +9,7 @@ describe('app', function(){
   it('should handle socket errors', function(done){
     const app = new Koa();
 
-    app.use(function *(ctx, next){
+    app.use(function(ctx, next){
       // triggers ctx.socket.writable == false
       ctx.socket.emit('error', new Error('boom'));
     });
@@ -27,7 +27,7 @@ describe('app', function(){
   it('should not .writeHead when !socket.writable', function(done){
     const app = new Koa();
 
-    app.use(function *(ctx, next){
+    app.use(function(ctx, next){
       // set .writable to false
       ctx.socket.writable = false;
       ctx.status = 204;
diff --git a/test/application/request.js b/test/application/request.js
index 3050127..3f85cc1 100644
--- a/test/application/request.js
+++ b/test/application/request.js
@@ -11,7 +11,7 @@ describe('app.request', function(){
   const app2 = new Koa();
 
   it('should merge properties', function(done){
-    app1.use(function *(ctx, next){
+    app1.use(function(ctx, next){
       assert.equal(ctx.request.message, 'hello');
       ctx.status = 204;
     });
@@ -22,7 +22,7 @@ describe('app.request', function(){
   });
 
   it('should not affect the original prototype', function(done){
-    app2.use(function *(ctx, next){
+    app2.use(function(ctx, next){
       assert.equal(ctx.request.message, undefined);
       ctx.status = 204;
     });
diff --git a/test/application/respond.js b/test/application/respond.js
index 982367b..bb7d640 100644
--- a/test/application/respond.js
+++ b/test/application/respond.js
@@ -9,10 +9,10 @@ const fs = require('fs');
 
 describe('app.respond', function(){
   describe('when ctx.respond === false', function(){
-    it('should function *(ctx)', function(done){
+    it('should function (ctx)', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = 'Hello';
         ctx.respond = false;
 
@@ -38,7 +38,7 @@ describe('app.respond', function(){
     it('should not send Content-Type header', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = '';
         ctx.type = null;
       });
@@ -60,7 +60,7 @@ describe('app.respond', function(){
     it('should not respond with the body', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = 'Hello';
       });
 
@@ -81,7 +81,7 @@ describe('app.respond', function(){
     it('should keep json headers', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = { hello: 'world' };
       });
 
@@ -102,7 +102,7 @@ describe('app.respond', function(){
     it('should keep string headers', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = 'hello world';
       });
 
@@ -123,7 +123,7 @@ describe('app.respond', function(){
     it('should keep buffer headers', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = new Buffer('hello world');
       });
 
@@ -144,7 +144,7 @@ describe('app.respond', function(){
     it('should respond with a 404 if no body was set', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
 
       });
 
@@ -158,7 +158,7 @@ describe('app.respond', function(){
     it('should respond with a 200 if body = ""', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = '';
       });
 
@@ -172,7 +172,7 @@ describe('app.respond', function(){
     it('should not overwrite the content-type', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.status = 200;
         ctx.type = 'application/javascript';
       });
@@ -202,7 +202,7 @@ describe('app.respond', function(){
     it('should not cause an app error', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx, next){
+      app.use(function(ctx, next){
         const res = ctx.res;
         ctx.status = 200;
         res.setHeader('Content-Type', 'text/html');
@@ -229,7 +229,7 @@ describe('app.respond', function(){
     it('should send the right body', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx, next){
+      app.use(function(ctx, next){
         const res = ctx.res;
         ctx.status = 200;
         res.setHeader('Content-Type', 'text/html');
@@ -253,7 +253,7 @@ describe('app.respond', function(){
       it('should respond with the associated status message', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx){
+        app.use(function(ctx){
           ctx.status = 400;
         });
 
@@ -271,7 +271,7 @@ describe('app.respond', function(){
       it('should respond without a body', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx){
+        app.use(function(ctx){
           ctx.status = 204;
         });
 
@@ -294,7 +294,7 @@ describe('app.respond', function(){
       it('should respond without a body', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx){
+        app.use(function(ctx){
           ctx.status = 205;
         });
 
@@ -317,7 +317,7 @@ describe('app.respond', function(){
       it('should respond without a body', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx){
+        app.use(function(ctx){
           ctx.status = 304;
         });
 
@@ -341,7 +341,7 @@ describe('app.respond', function(){
         const app = new Koa();
         statuses['700'] = 'custom status';
 
-        app.use(function *(ctx){
+        app.use(function(ctx){
           ctx.status = 700;
         });
 
@@ -363,7 +363,7 @@ describe('app.respond', function(){
       it('should respond with the custom status message', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx){
+        app.use(function(ctx){
           ctx.status = 200;
           ctx.message = 'ok';
         });
@@ -386,7 +386,7 @@ describe('app.respond', function(){
       it('should respond with the status code number', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx){
+        app.use(function(ctx){
           ctx.res.statusCode = 701;
         });
 
@@ -404,7 +404,7 @@ describe('app.respond', function(){
     it('should respond 204 by default', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = null;
       });
 
@@ -425,7 +425,7 @@ describe('app.respond', function(){
     it('should respond 204 with status=200', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.status = 200;
         ctx.body = null;
       });
@@ -447,7 +447,7 @@ describe('app.respond', function(){
     it('should respond 205 with status=205', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.status = 205;
         ctx.body = null;
       });
@@ -469,7 +469,7 @@ describe('app.respond', function(){
     it('should respond 304 with status=304', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.status = 304;
         ctx.body = null;
       });
@@ -493,7 +493,7 @@ describe('app.respond', function(){
     it('should respond', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = 'Hello';
       });
 
@@ -509,7 +509,7 @@ describe('app.respond', function(){
     it('should respond', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = new Buffer('Hello');
       });
 
@@ -525,7 +525,7 @@ describe('app.respond', function(){
     it('should respond', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = fs.createReadStream('package.json');
         ctx.set('Content-Type', 'application/json; charset=utf-8');
       });
@@ -547,7 +547,7 @@ describe('app.respond', function(){
     it('should strip content-length when overwriting', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = 'hello';
         ctx.body = fs.createReadStream('package.json');
         ctx.set('Content-Type', 'application/json; charset=utf-8');
@@ -570,7 +570,7 @@ describe('app.respond', function(){
     it('should keep content-length if not overwritten', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.length = fs.readFileSync('package.json').length;
         ctx.body = fs.createReadStream('package.json');
         ctx.set('Content-Type', 'application/json; charset=utf-8');
@@ -593,7 +593,7 @@ describe('app.respond', function(){
     it('should keep content-length if overwritten with the same stream', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.length = fs.readFileSync('package.json').length;
         const stream = fs.createReadStream('package.json');
         ctx.body = stream;
@@ -618,7 +618,7 @@ describe('app.respond', function(){
     it('should handle errors', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.set('Content-Type', 'application/json; charset=utf-8');
         ctx.body = fs.createReadStream('does not exist');
       });
@@ -635,7 +635,7 @@ describe('app.respond', function(){
     it('should handle errors when no content status', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.status = 204;
         ctx.body = fs.createReadStream('does not exist');
       });
@@ -650,7 +650,7 @@ describe('app.respond', function(){
     it('should handle all intermediate stream body errors', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = fs.createReadStream('does not exist');
         ctx.body = fs.createReadStream('does not exist');
         ctx.body = fs.createReadStream('does not exist');
@@ -668,7 +668,7 @@ describe('app.respond', function(){
     it('should respond with json', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = { hello: 'world' };
       });
 
@@ -685,7 +685,7 @@ describe('app.respond', function(){
     it('should emit "error" on the app', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         throw new Error('boom');
       });
 
@@ -703,7 +703,7 @@ describe('app.respond', function(){
       it('should expose the message', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx){
+        app.use(function(ctx){
           const err = new Error('sorry!');
           err.status = 403;
           err.expose = true;
@@ -721,7 +721,7 @@ describe('app.respond', function(){
       it('should respond with .status', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx){
+        app.use(function(ctx){
           const err = new Error('s3 explodes');
           err.status = 403;
           throw err;
@@ -737,7 +737,7 @@ describe('app.respond', function(){
     it('should respond with 500', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         throw new Error('boom!');
       });
 
@@ -752,16 +752,15 @@ describe('app.respond', function(){
     it('should be catchable', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx, next){
-        try {
-          yield next();
+      app.use(function(ctx, next){
+        return next().then(function(){
           ctx.body = 'Hello';
-        } catch (err) {
+        }).catch(function(){
           ctx.body = 'Got error';
-        }
+        });
       });
 
-      app.use(function *(ctx, next){
+      app.use(function(ctx, next){
         throw new Error('boom!');
       });
 
@@ -778,7 +777,7 @@ describe('app.respond', function(){
     it('should 200', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.status = 304;
         ctx.body = 'hello';
         ctx.status = 200;
@@ -795,7 +794,7 @@ describe('app.respond', function(){
     it('should 204', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.status = 200;
         ctx.body = 'hello';
         ctx.set('content-type', 'text/plain; charset=utf8');
diff --git a/test/application/response.js b/test/application/response.js
index e4e0d41..29b365c 100644
--- a/test/application/response.js
+++ b/test/application/response.js
@@ -11,7 +11,7 @@ describe('app.response', function(){
   const app2 = new Koa();
 
   it('should merge properties', function(done){
-    app1.use(function *(ctx, next){
+    app1.use(function(ctx, next){
       assert.equal(ctx.response.msg, 'hello');
       ctx.status = 204;
     });
@@ -22,7 +22,7 @@ describe('app.response', function(){
   });
 
   it('should not affect the original prototype', function(done){
-    app2.use(function *(ctx, next){
+    app2.use(function(ctx, next){
       assert.equal(ctx.response.msg, undefined);
       ctx.status = 204;
     });
diff --git a/test/application/use.js b/test/application/use.js
index a8fbfdf..fd7e948 100644
--- a/test/application/use.js
+++ b/test/application/use.js
@@ -9,22 +9,25 @@ describe('app.use(fn)', function(){
     const app = new Koa();
     const calls = [];
 
-    app.use(function *(ctx, next){
+    app.use(function(ctx, next){
       calls.push(1);
-      yield next();
-      calls.push(6);
+      return next().then(function(){
+        calls.push(6);
+      });
     });
 
-    app.use(function *(ctx, next){
+    app.use(function(ctx, next){
       calls.push(2);
-      yield next();
-      calls.push(5);
+      return next().then(function(){
+        calls.push(5);
+      });
     });
 
-    app.use(function *(ctx, next){
+    app.use(function(ctx, next){
       calls.push(3);
-      yield next();
-      calls.push(4);
+      return next().then(function(){
+        calls.push(4);
+      });
     });
 
     const server = app.listen();
@@ -59,4 +62,10 @@ describe('app.use(fn)', function(){
     (() => app.use('not a function')).should.throw('middleware must be a function!');
     done();
   });
+
+  it('should throw error for generator', function(){
+    const app = new Koa();
+
+    (() => app.use(function *(){})).should.throw('Support for generators has been removed. Use Promises or wrap your generator with co.wrap');
+  });
 });
diff --git a/test/context/cookies.js b/test/context/cookies.js
index 38957f0..a8641de 100644
--- a/test/context/cookies.js
+++ b/test/context/cookies.js
@@ -8,7 +8,7 @@ describe('ctx.cookies.set()', function(){
   it('should set an unsigned cookie', function(done){
     const app = new Koa();
 
-    app.use(function *(ctx, next){
+    app.use(function(ctx, next){
       ctx.cookies.set('name', 'jon');
       ctx.status = 204;
     });
@@ -33,7 +33,7 @@ describe('ctx.cookies.set()', function(){
       it('should error', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx, next){
+        app.use(function(ctx, next){
           try {
             ctx.cookies.set('foo', 'bar', { signed: true });
           } catch (err) {
@@ -52,7 +52,7 @@ describe('ctx.cookies.set()', function(){
 
       app.keys = ['a', 'b'];
 
-      app.use(function *(ctx, next){
+      app.use(function(ctx, next){
         ctx.cookies.set('name', 'jon', { signed: true });
         ctx.status = 204;
       });
diff --git a/test/context/onerror.js b/test/context/onerror.js
index 75b3171..b8e66a2 100644
--- a/test/context/onerror.js
+++ b/test/context/onerror.js
@@ -8,7 +8,7 @@ describe('ctx.onerror(err)', function(){
   it('should respond', function(done){
     const app = new Koa();
 
-    app.use(function *(ctx, next){
+    app.use(function(ctx, next){
       ctx.body = 'something else';
 
       ctx.throw(418, 'boom');
@@ -27,7 +27,7 @@ describe('ctx.onerror(err)', function(){
   it('should unset all headers', function(done){
     const app = new Koa();
 
-    app.use(function *(ctx, next){
+    app.use(function(ctx, next){
       ctx.set('Vary', 'Accept-Encoding');
       ctx.set('X-CSRF-Token', 'asdf');
       ctx.body = 'response';
@@ -57,7 +57,7 @@ describe('ctx.onerror(err)', function(){
       it('should respond 500', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx, next){
+        app.use(function(ctx, next){
           ctx.body = 'something else';
           const err = new Error('some error');
           err.status = 'notnumber';
@@ -78,7 +78,7 @@ describe('ctx.onerror(err)', function(){
       it('should respond 500', function(done){
         const app = new Koa();
 
-        app.use(function *(ctx, next){
+        app.use(function(ctx, next){
           ctx.body = 'something else';
           const err = new Error('some error');
           err.status = 9999;
@@ -100,7 +100,7 @@ describe('ctx.onerror(err)', function(){
     it('should response non-error thrown message', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx, next){
+      app.use(function(ctx, next){
         throw 'string error'; // eslint-disable-line no-throw-literal
       });
 
diff --git a/test/context/state.js b/test/context/state.js
index 7f316d3..b6bea98 100644
--- a/test/context/state.js
+++ b/test/context/state.js
@@ -9,7 +9,7 @@ describe('ctx.state', function(){
   it('should provide a ctx.state namespace', function(done){
     const app = new Koa();
 
-    app.use(function *(ctx){
+    app.use(function(ctx){
       assert.deepEqual(ctx.state, {});
     });
 
diff --git a/test/request/href.js b/test/request/href.js
index d5524a7..aaeaa71 100644
--- a/test/request/href.js
+++ b/test/request/href.js
@@ -26,7 +26,7 @@ describe('ctx.href', function(){
 
   it('should work with `GET http://example.com/foo`', function(done){
     const app = new Koa();
-    app.use(function *(ctx){
+    app.use(function(ctx){
       ctx.body = ctx.href;
     });
     app.listen(function(){
diff --git a/test/response/attachment.js b/test/response/attachment.js
index 7b73d2b..38d393e 100644
--- a/test/response/attachment.js
+++ b/test/response/attachment.js
@@ -34,7 +34,7 @@ describe('ctx.attachment([filename])', function(){
     it('should work with http client', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx, next){
+      app.use(function(ctx, next){
         ctx.attachment('path/to/include-no-ascii-char-中文名-ok.json');
         ctx.body = {foo: 'bar'};
       });
diff --git a/test/response/status.js b/test/response/status.js
index 6900645..b00b2b6 100644
--- a/test/response/status.js
+++ b/test/response/status.js
@@ -62,7 +62,7 @@ describe('res.status=', function(){
     it('should strip content related header fields', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.body = { foo: 'bar' };
         ctx.set('Content-Type', 'application/json; charset=utf-8');
         ctx.set('Content-Length', '15');
@@ -88,7 +88,7 @@ describe('res.status=', function(){
     it('should strip content releated header fields after status set', function(done){
       const app = new Koa();
 
-      app.use(function *(ctx){
+      app.use(function(ctx){
         ctx.status = status;
         ctx.body = { foo: 'bar' };
         ctx.set('Content-Type', 'application/json; charset=utf-8');