diff --git a/src/Subject.ts b/src/Subject.ts
index 8c628e7bb4..a8c5626ab5 100644
--- a/src/Subject.ts
+++ b/src/Subject.ts
@@ -25,7 +25,7 @@ export default class Subject<T> extends Observable<T> implements Observer<T>, Su
     return new BidirectionalSubject(source, destination);
   }
 
-  destination: Observer<T>;
+  protected destination: Observer<T>;
 
   observers: Observer<T>[] = [];
   isUnsubscribed: boolean = false;
@@ -41,8 +41,7 @@ export default class Subject<T> extends Observable<T> implements Observer<T>, Su
     return subject;
   }
 
-  _subscribe(subscriber: Subscriber<any>) : Subscription<T> {
-
+  _subscribe(subscriber: Subscriber<any>): Subscription<T> {
     if (subscriber.isUnsubscribed) {
       return;
     } else if (this.errorSignal) {
@@ -73,8 +72,7 @@ export default class Subject<T> extends Observable<T> implements Observer<T>, Su
     subscriptionUnsubscribe.call(this);
   }
 
-  next(value) {
-
+  next(value: T): void {
     if (this.isUnsubscribed) {
       return;
     }
@@ -90,25 +88,23 @@ export default class Subject<T> extends Observable<T> implements Observer<T>, Su
     }
   }
 
-  error(error) {
-
+  error(err?: any): void {
     if (this.isUnsubscribed || this.completeSignal) {
       return;
     }
 
     this.errorSignal = true;
-    this.errorInstance = error;
+    this.errorInstance = err;
 
     if (this.dispatching) {
       return;
     }
 
-    this._error(error);
+    this._error(err);
     this.unsubscribe();
   }
 
-  complete() {
-
+  complete(): void {
     if (this.isUnsubscribed || this.errorSignal) {
       return;
     }
@@ -124,7 +120,7 @@ export default class Subject<T> extends Observable<T> implements Observer<T>, Su
   }
 
 
-  _next(value?) {
+  _next(value: T): void {
     let index = -1;
     const observers = this.observers.slice(0);
     const len = observers.length;
@@ -134,7 +130,7 @@ export default class Subject<T> extends Observable<T> implements Observer<T>, Su
     }
   }
 
-  _error(error?) {
+  _error(err: any): void{
     let index = -1;
     const observers = this.observers;
     const len = observers.length;
@@ -144,13 +140,13 @@ export default class Subject<T> extends Observable<T> implements Observer<T>, Su
     this.isUnsubscribed = true;
 
     while(++index < len) {
-      observers[index].error(error);
+      observers[index].error(err);
     }
 
     this.isUnsubscribed = false;
   }
 
-  _complete() {
+  _complete(): void{
     let index = -1;
     const observers = this.observers;
     const len = observers.length;
@@ -168,39 +164,36 @@ export default class Subject<T> extends Observable<T> implements Observer<T>, Su
 }
 
 class BidirectionalSubject<T> extends Subject<T> {
-
-  constructor(source: Observable<any>, destination: Observer<any>) {
+  constructor(public source: Observable<any>, protected destination: Observer<any>) {
     super();
-    this.source = source;
-    this.destination = destination;
   }
 
-  _subscribe(subscriber: Subscriber<T>) {
+  _subscribe(subscriber: Subscriber<T>): Subscription<T> {
     const operator = this.operator;
     return this.source._subscribe.call(this.source, operator ? operator.call(subscriber) : subscriber);
   }
 
-  next(x) {
-    subscriberNext.call(this, x);
+  next(value?: T): void {
+    subscriberNext.call(this, value);
   }
 
-  error(e) {
-    subscriberError.call(this, e);
+  error(err?: any): void {
+    subscriberError.call(this, err);
   }
 
-  complete() {
+  complete(): void {
     subscriberComplete.call(this);
   }
 
-  _next(x) {
-    _subscriberNext.call(this, x);
+  _next(value: T): void {
+    _subscriberNext.call(this, value);
   }
 
-  _error(e) {
-    _subscriberError.call(this, e);
+  _error(err: any): void {
+    _subscriberError.call(this, err);
   }
 
-  _complete() {
+  _complete(): void {
     _subscriberComplete.call(this);
   }
 }
\ No newline at end of file
diff --git a/src/Subscriber.ts b/src/Subscriber.ts
index 68a50d8add..086afd2e7b 100644
--- a/src/Subscriber.ts
+++ b/src/Subscriber.ts
@@ -100,9 +100,9 @@ export default class Subscriber<T> extends Subscription<T> implements Observer<T
     }
   }
 
-  error(error?: any): void {
+  error(err?: any): void {
     if (!this.isUnsubscribed) {
-      this._error(error);
+      this._error(err);
       this.unsubscribe();
     }
   }
diff --git a/src/operators/groupBy.ts b/src/operators/groupBy.ts
index 210640eb42..c618752a7b 100644
--- a/src/operators/groupBy.ts
+++ b/src/operators/groupBy.ts
@@ -1,4 +1,5 @@
 import Subscriber from '../Subscriber';
+import Subscription from '../Subscription';
 import Observable from '../Observable';
 import Subject from '../Subject';
 import Map from '../util/Map';
@@ -22,7 +23,7 @@ export class GroupByObservable<T, R> extends Observable<GroupedObservable<R>> {
     super();
   }
 
-  _subscribe(subscriber) {
+  _subscribe(subscriber: Subscriber<any>): Subscription<T> | Function | void {
     const refCountSubscription = new RefCountSubscription();
     const groupBySubscriber = new GroupBySubscriber(
       subscriber, refCountSubscription, this.keySelector, this.elementSelector, this.durationSelector
@@ -45,7 +46,7 @@ class GroupBySubscriber<T, R> extends Subscriber<T> {
     this.add(destination);
   }
 
-  _next(x: T) {
+  _next(x: T): void {
     let key = tryCatch(this.keySelector)(x);
     if (key === errorObject) {
       this.error(key.e);
@@ -58,7 +59,7 @@ class GroupBySubscriber<T, R> extends Subscriber<T> {
         groups = this.groups = typeof key === 'string' ? new FastMap() : new Map();
       }
 
-      let group: Subject<R> = groups.get(key);
+      let group: Subject<T|R> = groups.get(key);
 
       if (!group) {
         groups.set(key, group = new Subject());
@@ -89,7 +90,7 @@ class GroupBySubscriber<T, R> extends Subscriber<T> {
     }
   }
 
-  _error(err: any) {
+  _error(err: any): void {
     const groups = this.groups;
     if (groups) {
       groups.forEach((group, key) => {
@@ -100,7 +101,7 @@ class GroupBySubscriber<T, R> extends Subscriber<T> {
     this.destination.error(err);
   }
 
-  _complete() {
+  _complete(): void {
     const groups = this.groups;
     if (groups) {
       groups.forEach((group, key) => {
@@ -111,7 +112,7 @@ class GroupBySubscriber<T, R> extends Subscriber<T> {
     this.destination.complete();
   }
 
-  removeGroup(key: string) {
+  removeGroup(key: string): void {
     this.groups.delete(key);
   }
 }
@@ -123,17 +124,17 @@ class GroupDurationSubscriber<T> extends Subscriber<T> {
     super(null);
   }
 
-  _next(value: T) {
+  _next(value: T): void {
     this.group.complete();
     this.parent.removeGroup(this.key);
   }
 
-  _error(err: any) {
+  _error(err: any): void {
     this.group.error(err);
     this.parent.removeGroup(this.key);
   }
 
-  _complete() {
+  _complete(): void {
     this.group.complete();
     this.parent.removeGroup(this.key);
   }
diff --git a/src/subjects/BehaviorSubject.ts b/src/subjects/BehaviorSubject.ts
index b69fe7a141..60a4202f40 100644
--- a/src/subjects/BehaviorSubject.ts
+++ b/src/subjects/BehaviorSubject.ts
@@ -3,7 +3,7 @@ import Subscriber from '../Subscriber';
 import Subscription from '../Subscription';
 
 export default class BehaviorSubject<T> extends Subject<T> {
-  constructor(public value: any) {
+  constructor(private value: any) {
     super();
   }
 
@@ -17,7 +17,7 @@ export default class BehaviorSubject<T> extends Subject<T> {
     return subscription;
   }
 
-  _next(value?) {
+  _next(value: T): void {
     super._next(this.value = value);
   }
 }
\ No newline at end of file
diff --git a/src/subjects/ReplaySubject.ts b/src/subjects/ReplaySubject.ts
index 9ce6cac7f1..875998b662 100644
--- a/src/subjects/ReplaySubject.ts
+++ b/src/subjects/ReplaySubject.ts
@@ -8,7 +8,7 @@ export default class ReplaySubject<T> extends Subject<T> {
   private bufferSize: number;
   private _windowTime: number;
   private scheduler: Scheduler;
-  private events: ReplayEvent<T> [] = [];
+  private events: ReplayEvent<T>[] = [];
 
   constructor(bufferSize: number = Number.POSITIVE_INFINITY,
               _windowTime: number = Number.POSITIVE_INFINITY,
@@ -19,7 +19,7 @@ export default class ReplaySubject<T> extends Subject<T> {
     this.scheduler = scheduler;
   }
 
-  _next(value?) {
+  _next(value: T): void {
     const now = this._getNow();
     this.events.push(new ReplayEvent(now, value));
     super._next(value);
@@ -35,12 +35,11 @@ export default class ReplaySubject<T> extends Subject<T> {
     return super._subscribe(subscriber);
   }
 
-  private _getNow() {
+  private _getNow(): number {
     return (this.scheduler || immediate).now();
   }
 
-  private _getEvents(now) {
-
+  private _getEvents(now): ReplayEvent<T>[] {
     const bufferSize = this.bufferSize;
     const _windowTime = this._windowTime;
     const events = this.events;
@@ -71,13 +70,7 @@ export default class ReplaySubject<T> extends Subject<T> {
 }
 
 class ReplayEvent<T> {
-
-  time: number;
-  value: T;
-
-  constructor(time: number, value: T) {
-    this.time = time;
-    this.value = value;
+  constructor(public time: number, public value: T) {
   }
 }