diff --git a/types/koa/index.d.ts b/types/koa/index.d.ts
index 4de24a8014..df6d5f82b9 100644
--- a/types/koa/index.d.ts
+++ b/types/koa/index.d.ts
@@ -3,6 +3,7 @@
 // Definitions by: DavidCai1993 <https://github.com/DavidCai1993>
 //                 jKey Lu <https://github.com/jkeylu>
 //                 Brice Bernard <https://github.com/brikou>
+//                 harryparkdotio <https://github.com/harryparkdotio>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -431,7 +432,10 @@ declare interface ContextDelegatedResponse {
     flushHeaders(): void;
 }
 
-declare class Application<StateT = any, CustomT = {}> extends EventEmitter {
+declare class Application<
+    StateT = Application.DefaultState,
+    CustomT = Application.DefaultContext
+> extends EventEmitter {
     proxy: boolean;
     middleware: Application.Middleware<StateT, CustomT>[];
     subdomainOffset: number;
@@ -512,7 +516,7 @@ declare class Application<StateT = any, CustomT = {}> extends EventEmitter {
      *
      * @api private
      */
-    createContext<StateT = any>(
+    createContext<StateT = Application.DefaultState>(
         req: IncomingMessage,
         res: ServerResponse,
     ): Application.ParameterizedContext<StateT>;
@@ -526,7 +530,22 @@ declare class Application<StateT = any, CustomT = {}> extends EventEmitter {
 }
 
 declare namespace Application {
-    type Middleware<StateT = any, CustomT = {}> = compose.Middleware<ParameterizedContext<StateT, CustomT>>;
+    type DefaultStateExtends = any;
+    /**
+     * This interface can be augmented by users to add types to Koa's default state
+     */
+    interface DefaultState extends DefaultStateExtends {}
+
+    type DefaultContextExtends = {};
+    /**
+     * This interface can be augmented by users to add types to Koa's default context
+     */
+    interface DefaultContext extends DefaultContextExtends {}
+
+    type Middleware<
+        StateT = DefaultState,
+        CustomT = DefaultContext
+    > = compose.Middleware<ParameterizedContext<StateT, CustomT>>;
 
     interface BaseRequest extends ContextDelegatedRequest {
         /**
@@ -701,7 +720,10 @@ declare namespace Application {
         respond?: boolean;
     }
 
-    type ParameterizedContext<StateT = any, CustomT = {}> = ExtendableContext & {
+    type ParameterizedContext<
+        StateT = DefaultState,
+        CustomT = DefaultContext
+    > = ExtendableContext & {
         state: StateT;
     } & CustomT;
 
diff --git a/types/koa/test/default.ts b/types/koa/test/default.ts
new file mode 100644
index 0000000000..dfc4ca68cf
--- /dev/null
+++ b/types/koa/test/default.ts
@@ -0,0 +1,55 @@
+import Koa = require('koa');
+
+declare module 'koa' {
+    interface DefaultState {
+        stateProperty: boolean;
+    }
+
+    interface DefaultContext {
+        logger: {
+            info: Function;
+            log: Function;
+            error: Function;
+        };
+    }
+}
+
+const app = new Koa();
+
+app.context.logger = {
+    info: () => {},
+    log: () => {},
+    error: () => {},
+};
+
+app.use((ctx, next) => {
+    ctx.state.stateProperty = false;
+    return next();
+})
+
+app.use<{ a: boolean }>(async (ctx, next) => {
+    ctx.state.a = true;
+    ctx.state.b = ''; // undeclared property
+    await next();
+});
+
+app.use((ctx, next) => {
+    const start: any = new Date();
+    return next().then(() => {
+        const end: any = new Date();
+        const ms = end - start;
+        ctx.logger.info(`${ctx.method} ${ctx.url} - ${ms}ms`);
+        ctx.user = {}; // undeclared property
+        ctx.assert(true, 404, 'Yep!');
+    });
+});
+
+// response
+app.use(ctx => {
+    ctx.body = 'Hello World';
+    ctx.body = ctx.URL.toString();
+});
+
+app.listen(3000);
+
+const server = app.listen();
diff --git a/types/koa/koa-tests.ts b/types/koa/test/index.ts
similarity index 100%
rename from types/koa/koa-tests.ts
rename to types/koa/test/index.ts
diff --git a/types/koa/tsconfig.json b/types/koa/tsconfig.json
index 5c4ba0f656..7c18151c2e 100644
--- a/types/koa/tsconfig.json
+++ b/types/koa/tsconfig.json
@@ -1,7 +1,8 @@
 {
     "files": [
         "index.d.ts",
-        "koa-tests.ts"
+        "test/index.ts",
+        "test/default.ts"
     ],
     "compilerOptions": {
         "module": "commonjs",