diff --git a/docs/api/index.md b/docs/api/index.md
index b8b7ebe..010d4d4 100644
--- a/docs/api/index.md
+++ b/docs/api/index.md
@@ -112,9 +112,22 @@ app.listen(3000);
   the following are supported:
 
   - `app.env` defaulting to the __NODE_ENV__ or "development"
+  - `app.keys` array of signed cookie keys
   - `app.proxy` when true proxy header fields will be trusted
   - `app.subdomainOffset` offset of `.subdomains` to ignore [2]
 
+  You can pass the settings to the constructor:
+  ```js
+  const Koa = require('koa');
+  const app = new Koa({ proxy: true });
+  ```
+  or dynamically:
+  ```js
+  const Koa = require('koa');
+  const app = new Koa();
+  app.proxy = true;
+  ```
+
 ## app.listen(...)
 
   A Koa application is not a 1-to-1 representation of an HTTP server.
diff --git a/lib/application.js b/lib/application.js
index 1854f37..a1f607f 100644
--- a/lib/application.js
+++ b/lib/application.js
@@ -34,13 +34,23 @@ module.exports = class Application extends Emitter {
    * @api public
    */
 
-  constructor() {
-    super();
+  /**
+    *
+    * @param {object} [options] Application options
+    * @param {string} [options.env='development'] Environment
+    * @param {string[]} [options.keys] Signed cookie keys
+    * @param {boolean} [options.proxy] Trust proxy headers
+    * @param {number} [options.subdomainOffset] Subdomain offset
+    *
+    */
 
-    this.proxy = false;
+  constructor(options = {}) {
+    super();
+    this.proxy = options.proxy || false;
     this.middleware = [];
-    this.subdomainOffset = 2;
-    this.env = process.env.NODE_ENV || 'development';
+    this.subdomainOffset = options.subdomainOffset || 2;
+    this.env = options.env || process.env.NODE_ENV || 'development';
+    this.keys = options.keys || undefined;
     this.context = Object.create(context);
     this.request = Object.create(request);
     this.response = Object.create(response);
diff --git a/test/application/index.js b/test/application/index.js
index 51d76ba..ff27111 100644
--- a/test/application/index.js
+++ b/test/application/index.js
@@ -53,4 +53,28 @@ describe('app', () => {
     process.env.NODE_ENV = NODE_ENV;
     assert.equal(app.env, 'development');
   });
+
+  it('should set env from the constructor', () => {
+    const env = 'custom';
+    const app = new Koa({ env });
+    assert.strictEqual(app.env, env);
+  });
+
+  it('should set proxy flag from the constructor', () => {
+    const proxy = true;
+    const app = new Koa({ proxy });
+    assert.strictEqual(app.proxy, proxy);
+  });
+
+  it('should set signed cookie keys from the constructor', () => {
+    const keys = ['customkey'];
+    const app = new Koa({ keys });
+    assert.strictEqual(app.keys, keys);
+  });
+
+  it('should set subdomainOffset from the constructor', () => {
+    const subdomainOffset = 3;
+    const app = new Koa({ subdomainOffset });
+    assert.strictEqual(app.subdomainOffset, subdomainOffset);
+  });
 });