diff --git a/src/Stream.ts b/src/Stream.ts
index 8b19be7..773de79 100644
--- a/src/Stream.ts
+++ b/src/Stream.ts
@@ -8,6 +8,7 @@ import {DebugOperator} from './operator/DebugOperator';
 import {FoldOperator} from './operator/FoldOperator';
 import {LastOperator} from './operator/LastOperator';
 import {StartWithOperator} from './operator/StartWithOperator';
+import {FlattenOperator} from './operator/FlattenOperator';
 import {FlattenConcurrentlyOperator} from './operator/FlattenConcurrentlyOperator';
 import {
   CombineProducer,
@@ -166,6 +167,10 @@ export class Stream<T> implements Listener<T> {
     return new Stream<T>(new StartWithOperator(this, x));
   }
 
+  flatten<R, T extends Stream<R>>(): T {
+    return <T> new Stream<R>(new FlattenOperator(<Stream<Stream<R>>> (<any> this)));
+  }
+
   flattenConcurrently<R, T extends Stream<R>>(): T {
     return <T> new Stream<R>(new FlattenConcurrentlyOperator(<Stream<Stream<R>>> (<any> this)));
   }
diff --git a/src/operator/FlattenOperator.ts b/src/operator/FlattenOperator.ts
new file mode 100644
index 0000000..9134c79
--- /dev/null
+++ b/src/operator/FlattenOperator.ts
@@ -0,0 +1,107 @@
+import {Observer} from '../Observer';
+import {Operator} from '../Operator';
+import {Stream} from '../Stream';
+import {emptyObserver} from '../utils/emptyObserver';
+import {MapOperator} from './MapOperator';
+
+export class Inner<T> implements Observer<T> {
+  constructor(public out: Stream<T>,
+              public op: FlattenOperator<T>) {
+  }
+
+  next(t: T) {
+    this.out.next(t);
+  }
+
+  error(err: any) {
+    this.out.error(err);
+  }
+
+  end() {
+    this.op.curr = null;
+    this.op.less();
+  }
+}
+
+export class Outer<T> implements Observer<Stream<T>> {
+  constructor(public out: Stream<T>,
+              public op: FlattenOperator<T>) {
+  }
+
+  next(s: Stream<T>) {
+    this.op.cut();
+    (this.op.curr = s).subscribe(this.op.inner = new Inner(this.out, this.op));
+  }
+
+  error(err: any) {
+    this.out.error(err);
+  }
+
+  end() {
+    this.op.open = false;
+    this.op.less();
+  }
+}
+
+export class MapOuter<T> implements Observer<T> {
+  constructor(public out: Stream<T>,
+              public pr: (t: T) => Stream<T>, // pr = project
+              public op: FlattenOperator<T>) {
+  }
+
+  next(v: T) {
+    this.op.cut();
+    (this.op.curr = this.pr(v)).subscribe(this.op.inner = new Inner(this.out, this.op));
+  }
+
+  error(err: any) {
+    this.out.error(err);
+  }
+
+  end() {
+    this.op.open = false;
+    this.op.less();
+  }
+}
+
+export class FlattenOperator<T> implements Operator<Stream<T>, T> {
+  public proxy: Observer<T | Stream<T>> = emptyObserver;
+  public mapOp: MapOperator<T, Stream<T>>;
+  public curr: Stream<T>; // Current inner Stream
+  public inner: Observer<T>; // Current inner Observer
+  public open: boolean = true;
+  public out: Stream<T>;
+
+  constructor(public ins: Stream<Stream<T>>) {
+    if (ins._prod instanceof MapOperator) {
+      this.mapOp = <MapOperator<T, Stream<T>>> ins._prod;
+    }
+  }
+
+  start(out: Stream<T>): void {
+    this.out = out;
+    const mapOp = this.mapOp;
+    if (mapOp) {
+      mapOp.ins.subscribe(this.proxy = new MapOuter(out, mapOp.project, this));
+    } else {
+      this.ins.subscribe(this.proxy = new Outer(out, this));
+    }
+  }
+
+  stop(): void {
+    this.ins.unsubscribe(this.proxy);
+  }
+
+  cut(): void {
+    const {curr, inner} = this;
+    if (curr && inner) {
+      curr.unsubscribe(inner);
+    }
+  }
+
+  less(): void {
+    if (!this.open && !this.curr) {
+      this.out.end();
+    }
+  }
+}
diff --git a/tests/operator/flatten.ts b/tests/operator/flatten.ts
new file mode 100644
index 0000000..af4e4a1
--- /dev/null
+++ b/tests/operator/flatten.ts
@@ -0,0 +1,73 @@
+import xs from '../../src/index';
+import * as assert from 'assert';
+
+describe('Stream.prototype.flatten', () => {
+  describe('with map', () => {
+    it('should expand each interval event with 3 sync events', (done) => {
+      const stream = xs.interval(100).take(3)
+      .map(i => xs.of(1 + i, 2 + i, 3 + i))
+      .flatten();
+      const expected = [1, 2, 3, 2, 3, 4, 3, 4, 5];
+      const observer = {
+        next: (x: number) => {
+          assert.equal(x, expected.shift());
+          if (expected.length === 0) {
+            stream.unsubscribe(observer);
+            done();
+          }
+        },
+        error: done.fail,
+        end: done.fail,
+      };
+      stream.subscribe(observer);
+    });
+
+    it('should expand 3 sync events as an interval, only last one passes', (done) => {
+      const stream = xs.from([0, 1, 2])
+        .map(i => xs.interval(100 * i).take(2).map(x => `${i}${x}`))
+        .flatten();
+      // ---x---x---x---x---x---x
+      // ---00--01
+      // -------10------11
+      // -----------20----------21
+      const expected = ['20', '21'];
+      const observer = {
+        next: (x: number) => {
+          assert.equal(x, expected.shift());
+          if (expected.length === 0) {
+            stream.unsubscribe(observer);
+            done();
+          }
+        },
+        error: (err: any) => done(err),
+        end: () => done(new Error('No end() should be called')),
+      };
+      stream.subscribe(observer);
+    });
+
+    it('should expand 3 async events as an interval each', (done) => {
+      const stream = xs.interval(140).take(3)
+        .map(i =>
+          xs.interval(100 * (i < 2 ? 1 : i)).take(3).map(x => `${i}${x}`)
+        )
+        .flatten();
+      // ---x---x---x---x---x---x---x---x---x---x---x---x
+      // ---00--01--02
+      //      ----10--11--12
+      //           ------------20-----------21----------22
+      const expected = ['00', '10', '20', '21', '22'];
+      const observer = {
+        next: (x: number) => {
+          assert.equal(x, expected.shift());
+          if (expected.length === 0) {
+            stream.unsubscribe(observer);
+            done();
+          }
+        },
+        error: (err: any) => done(err),
+        end: () => done(new Error('No end() should be called')),
+      };
+      stream.subscribe(observer);
+    });
+  });
+});
diff --git a/tests/operator/flattenConcurrently.ts b/tests/operator/flattenConcurrently.ts
index 463461b..5070d5a 100644
--- a/tests/operator/flattenConcurrently.ts
+++ b/tests/operator/flattenConcurrently.ts
@@ -44,5 +44,30 @@ describe('Stream.prototype.flattenConcurrently', () => {
       };
       stream.addListener(listener);
     });
+
+    it('should expand 3 async events as an interval each', (done) => {
+      const stream = xs.interval(140).take(3)
+        .map(i =>
+          xs.interval(100 * (i < 2 ? 1 : i)).take(3).map(x => `${i}${x}`)
+        )
+        .flattenConcurrently();
+      // ---x---x---x---x---x---x---x---x---x---x---x---x
+      // ---00--01--02
+      //      ----10--11--12
+      //           ------------20-----------21----------22
+      const expected = ['00', '01', '10', '02', '11', '12', '20', '21', '22'];
+      const observer = {
+        next: (x: number) => {
+          assert.equal(x, expected.shift());
+          if (expected.length === 0) {
+            stream.unsubscribe(observer);
+            done();
+          }
+        },
+        error: (err: any) => done(err),
+        end: () => done(new Error('No end() should be called')),
+      };
+      stream.subscribe(observer);
+    });
   });
 });