diff --git a/src/main/java/io/reactivex/CompletableEmitter.java b/src/main/java/io/reactivex/CompletableEmitter.java
index 3943083988..a1bd1e69bf 100644
--- a/src/main/java/io/reactivex/CompletableEmitter.java
+++ b/src/main/java/io/reactivex/CompletableEmitter.java
@@ -13,6 +13,7 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import io.reactivex.disposables.Disposable;
 import io.reactivex.functions.Cancellable;
 
@@ -35,21 +36,21 @@ public interface CompletableEmitter {
      * Signal an exception.
      * @param t the exception, not null
      */
-    void onError(Throwable t);
+    void onError(@NonNull Throwable t);
 
     /**
      * Sets a Disposable on this emitter; any previous Disposable
      * or Cancellation will be disposed/cancelled.
      * @param d the disposable, null is allowed
      */
-    void setDisposable(Disposable d);
+    void setDisposable(@Nullable Disposable d);
 
     /**
      * Sets a Cancellable on this emitter; any previous Disposable
      * or Cancellation will be disposed/cancelled.
      * @param c the cancellable resource, null is allowed
      */
-    void setCancellable(Cancellable c);
+    void setCancellable(@Nullable Cancellable c);
 
     /**
      * Returns true if the downstream disposed the sequence.
diff --git a/src/main/java/io/reactivex/CompletableObserver.java b/src/main/java/io/reactivex/CompletableObserver.java
index 47c626e0e1..54695d9a41 100644
--- a/src/main/java/io/reactivex/CompletableObserver.java
+++ b/src/main/java/io/reactivex/CompletableObserver.java
@@ -13,6 +13,7 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.NonNull;
 import io.reactivex.disposables.Disposable;
 
 /**
@@ -24,7 +25,7 @@ public interface CompletableObserver {
      * then can be used to cancel the subscription at any time.
      * @param d the Disposable instance to call dispose on for cancellation, not null
      */
-    void onSubscribe(Disposable d);
+    void onSubscribe(@NonNull Disposable d);
 
     /**
      * Called once the deferred computation completes normally.
@@ -35,5 +36,5 @@ public interface CompletableObserver {
      * Called once if the deferred computation 'throws' an exception.
      * @param e the exception, not null.
      */
-    void onError(Throwable e);
+    void onError(@NonNull Throwable e);
 }
diff --git a/src/main/java/io/reactivex/CompletableOnSubscribe.java b/src/main/java/io/reactivex/CompletableOnSubscribe.java
index 1a403a5674..f4dae79f5d 100644
--- a/src/main/java/io/reactivex/CompletableOnSubscribe.java
+++ b/src/main/java/io/reactivex/CompletableOnSubscribe.java
@@ -12,6 +12,8 @@
  */
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * A functional interface that has a {@code subscribe()} method that receives
  * an instance of a {@link CompletableEmitter} instance that allows pushing
@@ -24,6 +26,6 @@ public interface CompletableOnSubscribe {
      * @param e the safe emitter instance, never null
      * @throws Exception on error
      */
-    void subscribe(CompletableEmitter e) throws Exception;
+    void subscribe(@NonNull CompletableEmitter e) throws Exception;
 }
 
diff --git a/src/main/java/io/reactivex/CompletableOperator.java b/src/main/java/io/reactivex/CompletableOperator.java
index d7edcfb454..749e41fa05 100644
--- a/src/main/java/io/reactivex/CompletableOperator.java
+++ b/src/main/java/io/reactivex/CompletableOperator.java
@@ -13,6 +13,8 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Interface to map/wrap a downstream observer to an upstream observer.
  */
@@ -23,5 +25,6 @@ public interface CompletableOperator {
      * @return the parent CompletableObserver instance
      * @throws Exception on failure
      */
-    CompletableObserver apply(CompletableObserver observer) throws Exception;
+    @NonNull
+    CompletableObserver apply(@NonNull CompletableObserver observer) throws Exception;
 }
diff --git a/src/main/java/io/reactivex/CompletableSource.java b/src/main/java/io/reactivex/CompletableSource.java
index 09d41d58f6..57019f3164 100644
--- a/src/main/java/io/reactivex/CompletableSource.java
+++ b/src/main/java/io/reactivex/CompletableSource.java
@@ -12,6 +12,8 @@
  */
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Represents a basic {@link Completable} source base interface,
  * consumable via an {@link CompletableObserver}.
@@ -25,5 +27,5 @@ public interface CompletableSource {
      * @param cs the CompletableObserver, not null
      * @throws NullPointerException if {@code cs} is null
      */
-    void subscribe(CompletableObserver cs);
+    void subscribe(@NonNull CompletableObserver cs);
 }
diff --git a/src/main/java/io/reactivex/CompletableTransformer.java b/src/main/java/io/reactivex/CompletableTransformer.java
index 15948f8112..f6569916ef 100644
--- a/src/main/java/io/reactivex/CompletableTransformer.java
+++ b/src/main/java/io/reactivex/CompletableTransformer.java
@@ -13,6 +13,8 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Convenience interface and callback used by the compose operator to turn a Completable into another
  * Completable fluently.
@@ -23,5 +25,6 @@ public interface CompletableTransformer {
      * @param upstream the upstream Completable instance
      * @return the transformed CompletableSource instance
      */
-    CompletableSource apply(Completable upstream);
+    @NonNull
+    CompletableSource apply(@NonNull Completable upstream);
 }
diff --git a/src/main/java/io/reactivex/FlowableEmitter.java b/src/main/java/io/reactivex/FlowableEmitter.java
index 12eff99923..855ff08b6b 100644
--- a/src/main/java/io/reactivex/FlowableEmitter.java
+++ b/src/main/java/io/reactivex/FlowableEmitter.java
@@ -13,6 +13,7 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import io.reactivex.disposables.Disposable;
 import io.reactivex.functions.Cancellable;
 
@@ -35,14 +36,14 @@ public interface FlowableEmitter<T> extends Emitter<T> {
      * or Cancellation will be unsubscribed/cancelled.
      * @param s the disposable, null is allowed
      */
-    void setDisposable(Disposable s);
+    void setDisposable(@Nullable Disposable s);
 
     /**
      * Sets a Cancellable on this emitter; any previous Disposable
      * or Cancellation will be unsubscribed/cancelled.
      * @param c the cancellable resource, null is allowed
      */
-    void setCancellable(Cancellable c);
+    void setCancellable(@Nullable Cancellable c);
 
     /**
      * The current outstanding request amount.
@@ -62,5 +63,6 @@ public interface FlowableEmitter<T> extends Emitter<T> {
      * Ensures that calls to onNext, onError and onComplete are properly serialized.
      * @return the serialized FlowableEmitter
      */
+    @NonNull
     FlowableEmitter<T> serialize();
 }
diff --git a/src/main/java/io/reactivex/FlowableOnSubscribe.java b/src/main/java/io/reactivex/FlowableOnSubscribe.java
index 50f0c0ab09..40911eb3cd 100644
--- a/src/main/java/io/reactivex/FlowableOnSubscribe.java
+++ b/src/main/java/io/reactivex/FlowableOnSubscribe.java
@@ -12,6 +12,8 @@
  */
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * A functional interface that has a {@code subscribe()} method that receives
  * an instance of a {@link FlowableEmitter} instance that allows pushing
@@ -26,6 +28,6 @@ public interface FlowableOnSubscribe<T> {
      * @param e the safe emitter instance, never null
      * @throws Exception on error
      */
-    void subscribe(FlowableEmitter<T> e) throws Exception;
+    void subscribe(@NonNull FlowableEmitter<T> e) throws Exception;
 }
 
diff --git a/src/main/java/io/reactivex/FlowableOperator.java b/src/main/java/io/reactivex/FlowableOperator.java
index d7b803103b..4211a3ab52 100644
--- a/src/main/java/io/reactivex/FlowableOperator.java
+++ b/src/main/java/io/reactivex/FlowableOperator.java
@@ -13,6 +13,7 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import org.reactivestreams.Subscriber;
 
 /**
@@ -28,5 +29,6 @@ public interface FlowableOperator<Downstream, Upstream> {
      * @return the parent Subscriber instance
      * @throws Exception on failure
      */
-    Subscriber<? super Upstream> apply(Subscriber<? super Downstream> observer) throws Exception;
+    @NonNull
+    Subscriber<? super Upstream> apply(@NonNull Subscriber<? super Downstream> observer) throws Exception;
 }
diff --git a/src/main/java/io/reactivex/FlowableSubscriber.java b/src/main/java/io/reactivex/FlowableSubscriber.java
index dabd898dae..8076c5c143 100644
--- a/src/main/java/io/reactivex/FlowableSubscriber.java
+++ b/src/main/java/io/reactivex/FlowableSubscriber.java
@@ -13,10 +13,9 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import org.reactivestreams.*;
 
-import io.reactivex.annotations.Experimental;
-
 /**
  * Represents a Reactive-Streams inspired Subscriber that is RxJava 2 only
  * and weakens rules §1.3 and §3.9 of the specification for gaining performance.
@@ -37,5 +36,5 @@ public interface FlowableSubscriber<T> extends Subscriber<T> {
      * {@inheritDoc}
      */
     @Override
-    void onSubscribe(Subscription s);
+    void onSubscribe(@NonNull Subscription s);
 }
diff --git a/src/main/java/io/reactivex/FlowableTransformer.java b/src/main/java/io/reactivex/FlowableTransformer.java
index f1ad26b5b3..78a0f43e9d 100644
--- a/src/main/java/io/reactivex/FlowableTransformer.java
+++ b/src/main/java/io/reactivex/FlowableTransformer.java
@@ -13,6 +13,7 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import org.reactivestreams.Publisher;
 
 /**
@@ -28,5 +29,6 @@ public interface FlowableTransformer<Upstream, Downstream> {
      * @param upstream the upstream Flowable instance
      * @return the transformed Publisher instance
      */
-    Publisher<Downstream> apply(Flowable<Upstream> upstream);
+    @NonNull
+    Publisher<Downstream> apply(@NonNull Flowable<Upstream> upstream);
 }
diff --git a/src/main/java/io/reactivex/MaybeEmitter.java b/src/main/java/io/reactivex/MaybeEmitter.java
index c05e4408f9..38389d250f 100644
--- a/src/main/java/io/reactivex/MaybeEmitter.java
+++ b/src/main/java/io/reactivex/MaybeEmitter.java
@@ -13,6 +13,7 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import io.reactivex.disposables.Disposable;
 import io.reactivex.functions.Cancellable;
 
@@ -32,13 +33,13 @@ public interface MaybeEmitter<T> {
      * Signal a success value.
      * @param t the value, not null
      */
-    void onSuccess(T t);
+    void onSuccess(@NonNull T t);
 
     /**
      * Signal an exception.
      * @param t the exception, not null
      */
-    void onError(Throwable t);
+    void onError(@NonNull Throwable t);
 
     /**
      * Signal the completion.
@@ -50,14 +51,14 @@ public interface MaybeEmitter<T> {
      * or Cancellation will be unsubscribed/cancelled.
      * @param s the disposable, null is allowed
      */
-    void setDisposable(Disposable s);
+    void setDisposable(@Nullable Disposable s);
 
     /**
      * Sets a Cancellable on this emitter; any previous Disposable
      * or Cancellation will be unsubscribed/cancelled.
      * @param c the cancellable resource, null is allowed
      */
-    void setCancellable(Cancellable c);
+    void setCancellable(@Nullable Cancellable c);
 
     /**
      * Returns true if the downstream cancelled the sequence.
diff --git a/src/main/java/io/reactivex/MaybeObserver.java b/src/main/java/io/reactivex/MaybeObserver.java
index d2d3b7e6a1..42a0ac7bfe 100644
--- a/src/main/java/io/reactivex/MaybeObserver.java
+++ b/src/main/java/io/reactivex/MaybeObserver.java
@@ -13,7 +13,9 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import io.reactivex.disposables.Disposable;
+
 /**
  * Provides a mechanism for receiving push-based notifications.
  * <p>
@@ -37,7 +39,7 @@ public interface MaybeObserver<T> {
      * @param d the Disposable instance whose {@link Disposable#dispose()} can
      * be called anytime to cancel the connection
      */
-    void onSubscribe(Disposable d);
+    void onSubscribe(@NonNull Disposable d);
 
     /**
      * Notifies the MaybeObserver with one item and that the {@link Maybe} has finished sending
@@ -48,7 +50,7 @@ public interface MaybeObserver<T> {
      * @param t
      *          the item emitted by the Maybe
      */
-    void onSuccess(T t);
+    void onSuccess(@NonNull T t);
 
     /**
      * Notifies the MaybeObserver that the {@link Maybe} has experienced an error condition.
@@ -58,7 +60,7 @@ public interface MaybeObserver<T> {
      * @param e
      *          the exception encountered by the Maybe
      */
-    void onError(Throwable e);
+    void onError(@NonNull Throwable e);
 
     /**
      * Called once the deferred computation completes normally.
diff --git a/src/main/java/io/reactivex/MaybeOnSubscribe.java b/src/main/java/io/reactivex/MaybeOnSubscribe.java
index 255bf8401a..9c03275797 100644
--- a/src/main/java/io/reactivex/MaybeOnSubscribe.java
+++ b/src/main/java/io/reactivex/MaybeOnSubscribe.java
@@ -12,6 +12,8 @@
  */
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * A functional interface that has a {@code subscribe()} method that receives
  * an instance of a {@link MaybeEmitter} instance that allows pushing
@@ -26,6 +28,6 @@ public interface MaybeOnSubscribe<T> {
      * @param e the safe emitter instance, never null
      * @throws Exception on error
      */
-    void subscribe(MaybeEmitter<T> e) throws Exception;
+    void subscribe(@NonNull MaybeEmitter<T> e) throws Exception;
 }
 
diff --git a/src/main/java/io/reactivex/MaybeOperator.java b/src/main/java/io/reactivex/MaybeOperator.java
index 0bd3ab15d6..9e7a54f122 100644
--- a/src/main/java/io/reactivex/MaybeOperator.java
+++ b/src/main/java/io/reactivex/MaybeOperator.java
@@ -10,9 +10,10 @@
  * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
  * the License for the specific language governing permissions and limitations under the License.
  */
-
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Interface to map/wrap a downstream observer to an upstream observer.
  *
@@ -26,5 +27,6 @@ public interface MaybeOperator<Downstream, Upstream> {
      * @return the parent MaybeObserver instance
      * @throws Exception on failure
      */
-    MaybeObserver<? super Upstream> apply(MaybeObserver<? super Downstream> observer) throws Exception;
+    @NonNull
+    MaybeObserver<? super Upstream> apply(@NonNull MaybeObserver<? super Downstream> observer) throws Exception;
 }
diff --git a/src/main/java/io/reactivex/MaybeSource.java b/src/main/java/io/reactivex/MaybeSource.java
index 697c51c745..4694eb0cef 100644
--- a/src/main/java/io/reactivex/MaybeSource.java
+++ b/src/main/java/io/reactivex/MaybeSource.java
@@ -12,6 +12,8 @@
  */
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Represents a basic {@link Maybe} source base interface,
  * consumable via an {@link MaybeObserver}.
@@ -29,5 +31,5 @@ public interface MaybeSource<T> {
      * @param observer the MaybeObserver, not null
      * @throws NullPointerException if {@code observer} is null
      */
-    void subscribe(MaybeObserver<? super T> observer);
+    void subscribe(@NonNull MaybeObserver<? super T> observer);
 }
diff --git a/src/main/java/io/reactivex/MaybeTransformer.java b/src/main/java/io/reactivex/MaybeTransformer.java
index 51874dec63..1526913b53 100644
--- a/src/main/java/io/reactivex/MaybeTransformer.java
+++ b/src/main/java/io/reactivex/MaybeTransformer.java
@@ -13,6 +13,8 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Interface to compose Maybes.
  *
@@ -26,5 +28,6 @@ public interface MaybeTransformer<Upstream, Downstream> {
      * @param upstream the upstream Maybe instance
      * @return the transformed MaybeSource instance
      */
-    MaybeSource<Downstream> apply(Maybe<Upstream> upstream);
+    @NonNull
+    MaybeSource<Downstream> apply(@NonNull Maybe<Upstream> upstream);
 }
diff --git a/src/main/java/io/reactivex/ObservableEmitter.java b/src/main/java/io/reactivex/ObservableEmitter.java
index 94f324e019..bf2c890038 100644
--- a/src/main/java/io/reactivex/ObservableEmitter.java
+++ b/src/main/java/io/reactivex/ObservableEmitter.java
@@ -13,6 +13,7 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import io.reactivex.disposables.Disposable;
 import io.reactivex.functions.Cancellable;
 
@@ -34,14 +35,14 @@ public interface ObservableEmitter<T> extends Emitter<T> {
      * or Cancellation will be unsubscribed/cancelled.
      * @param d the disposable, null is allowed
      */
-    void setDisposable(Disposable d);
+    void setDisposable(@Nullable Disposable d);
 
     /**
      * Sets a Cancellable on this emitter; any previous Disposable
      * or Cancellation will be unsubscribed/cancelled.
      * @param c the cancellable resource, null is allowed
      */
-    void setCancellable(Cancellable c);
+    void setCancellable(@Nullable Cancellable c);
 
     /**
      * Returns true if the downstream disposed the sequence.
@@ -53,5 +54,6 @@ public interface ObservableEmitter<T> extends Emitter<T> {
      * Ensures that calls to onNext, onError and onComplete are properly serialized.
      * @return the serialized ObservableEmitter
      */
+    @NonNull
     ObservableEmitter<T> serialize();
 }
diff --git a/src/main/java/io/reactivex/ObservableOnSubscribe.java b/src/main/java/io/reactivex/ObservableOnSubscribe.java
index 7c386e5fb4..6ed97c7779 100644
--- a/src/main/java/io/reactivex/ObservableOnSubscribe.java
+++ b/src/main/java/io/reactivex/ObservableOnSubscribe.java
@@ -12,6 +12,8 @@
  */
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * A functional interface that has a {@code subscribe()} method that receives
  * an instance of an {@link ObservableEmitter} instance that allows pushing
@@ -26,6 +28,6 @@ public interface ObservableOnSubscribe<T> {
      * @param e the safe emitter instance, never null
      * @throws Exception on error
      */
-    void subscribe(ObservableEmitter<T> e) throws Exception;
+    void subscribe(@NonNull ObservableEmitter<T> e) throws Exception;
 }
 
diff --git a/src/main/java/io/reactivex/ObservableOperator.java b/src/main/java/io/reactivex/ObservableOperator.java
index ad4df3e6ff..18ec3ecef6 100644
--- a/src/main/java/io/reactivex/ObservableOperator.java
+++ b/src/main/java/io/reactivex/ObservableOperator.java
@@ -13,6 +13,8 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Interface to map/wrap a downstream observer to an upstream observer.
  *
@@ -26,5 +28,6 @@ public interface ObservableOperator<Downstream, Upstream> {
      * @return the parent Observer instance
      * @throws Exception on failure
      */
-    Observer<? super Upstream> apply(Observer<? super Downstream> observer) throws Exception;
+    @NonNull
+    Observer<? super Upstream> apply(@NonNull Observer<? super Downstream> observer) throws Exception;
 }
diff --git a/src/main/java/io/reactivex/ObservableSource.java b/src/main/java/io/reactivex/ObservableSource.java
index f5890a1d9f..ea0b16490b 100644
--- a/src/main/java/io/reactivex/ObservableSource.java
+++ b/src/main/java/io/reactivex/ObservableSource.java
@@ -12,6 +12,8 @@
  */
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Represents a basic, non-backpressured {@link Observable} source base interface,
  * consumable via an {@link Observer}.
@@ -26,5 +28,5 @@ public interface ObservableSource<T> {
      * @param observer the Observer, not null
      * @throws NullPointerException if {@code observer} is null
      */
-    void subscribe(Observer<? super T> observer);
+    void subscribe(@NonNull Observer<? super T> observer);
 }
diff --git a/src/main/java/io/reactivex/ObservableTransformer.java b/src/main/java/io/reactivex/ObservableTransformer.java
index 1de17a7f69..a7f20eb965 100644
--- a/src/main/java/io/reactivex/ObservableTransformer.java
+++ b/src/main/java/io/reactivex/ObservableTransformer.java
@@ -13,6 +13,8 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Interface to compose Observables.
  *
@@ -26,5 +28,6 @@ public interface ObservableTransformer<Upstream, Downstream> {
      * @param upstream the upstream Observable instance
      * @return the transformed ObservableSource instance
      */
-    ObservableSource<Downstream> apply(Observable<Upstream> upstream);
+    @NonNull
+    ObservableSource<Downstream> apply(@NonNull Observable<Upstream> upstream);
 }
diff --git a/src/main/java/io/reactivex/SingleEmitter.java b/src/main/java/io/reactivex/SingleEmitter.java
index 8c49c4c79d..5e23c7d5a5 100644
--- a/src/main/java/io/reactivex/SingleEmitter.java
+++ b/src/main/java/io/reactivex/SingleEmitter.java
@@ -13,6 +13,7 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import io.reactivex.disposables.Disposable;
 import io.reactivex.functions.Cancellable;
 
@@ -32,27 +33,27 @@ public interface SingleEmitter<T> {
      * Signal a success value.
      * @param t the value, not null
      */
-    void onSuccess(T t);
+    void onSuccess(@NonNull T t);
 
     /**
      * Signal an exception.
      * @param t the exception, not null
      */
-    void onError(Throwable t);
+    void onError(@NonNull Throwable t);
 
     /**
      * Sets a Disposable on this emitter; any previous Disposable
      * or Cancellation will be unsubscribed/cancelled.
      * @param s the disposable, null is allowed
      */
-    void setDisposable(Disposable s);
+    void setDisposable(@Nullable Disposable s);
 
     /**
      * Sets a Cancellable on this emitter; any previous Disposable
      * or Cancellation will be unsubscribed/cancelled.
      * @param c the cancellable resource, null is allowed
      */
-    void setCancellable(Cancellable c);
+    void setCancellable(@Nullable Cancellable c);
 
     /**
      * Returns true if the downstream cancelled the sequence.
diff --git a/src/main/java/io/reactivex/SingleObserver.java b/src/main/java/io/reactivex/SingleObserver.java
index 9a81fc3134..eeda4af342 100644
--- a/src/main/java/io/reactivex/SingleObserver.java
+++ b/src/main/java/io/reactivex/SingleObserver.java
@@ -13,6 +13,7 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
 import io.reactivex.disposables.Disposable;
 
 /**
@@ -39,7 +40,7 @@ public interface SingleObserver<T> {
      * be called anytime to cancel the connection
      * @since 2.0
      */
-    void onSubscribe(Disposable d);
+    void onSubscribe(@NonNull Disposable d);
 
     /**
      * Notifies the SingleObserver with a single item and that the {@link Single} has finished sending
@@ -50,7 +51,7 @@ public interface SingleObserver<T> {
      * @param t
      *          the item emitted by the Single
      */
-    void onSuccess(T t);
+    void onSuccess(@NonNull T t);
 
     /**
      * Notifies the SingleObserver that the {@link Single} has experienced an error condition.
@@ -60,5 +61,5 @@ public interface SingleObserver<T> {
      * @param e
      *          the exception encountered by the Single
      */
-    void onError(Throwable e);
+    void onError(@NonNull Throwable e);
 }
diff --git a/src/main/java/io/reactivex/SingleOnSubscribe.java b/src/main/java/io/reactivex/SingleOnSubscribe.java
index a2ce2751ee..1f65502729 100644
--- a/src/main/java/io/reactivex/SingleOnSubscribe.java
+++ b/src/main/java/io/reactivex/SingleOnSubscribe.java
@@ -12,6 +12,8 @@
  */
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * A functional interface that has a {@code subscribe()} method that receives
  * an instance of a {@link SingleEmitter} instance that allows pushing
@@ -26,6 +28,6 @@ public interface SingleOnSubscribe<T> {
      * @param e the safe emitter instance, never null
      * @throws Exception on error
      */
-    void subscribe(SingleEmitter<T> e) throws Exception;
+    void subscribe(@NonNull SingleEmitter<T> e) throws Exception;
 }
 
diff --git a/src/main/java/io/reactivex/SingleOperator.java b/src/main/java/io/reactivex/SingleOperator.java
index a6b260e264..92ccfe7927 100644
--- a/src/main/java/io/reactivex/SingleOperator.java
+++ b/src/main/java/io/reactivex/SingleOperator.java
@@ -13,6 +13,8 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Interface to map/wrap a downstream observer to an upstream observer.
  *
@@ -26,5 +28,6 @@ public interface SingleOperator<Downstream, Upstream> {
      * @return the parent SingleObserver instance
      * @throws Exception on failure
      */
-    SingleObserver<? super Upstream> apply(SingleObserver<? super Downstream> observer) throws Exception;
+    @NonNull
+    SingleObserver<? super Upstream> apply(@NonNull SingleObserver<? super Downstream> observer) throws Exception;
 }
diff --git a/src/main/java/io/reactivex/SingleSource.java b/src/main/java/io/reactivex/SingleSource.java
index 421b9d921b..befdd5858a 100644
--- a/src/main/java/io/reactivex/SingleSource.java
+++ b/src/main/java/io/reactivex/SingleSource.java
@@ -12,6 +12,8 @@
  */
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Represents a basic {@link Single} source base interface,
  * consumable via an {@link SingleObserver}.
@@ -29,5 +31,5 @@ public interface SingleSource<T> {
      * @param observer the SingleObserver, not null
      * @throws NullPointerException if {@code observer} is null
      */
-    void subscribe(SingleObserver<? super T> observer);
+    void subscribe(@NonNull SingleObserver<? super T> observer);
 }
diff --git a/src/main/java/io/reactivex/SingleTransformer.java b/src/main/java/io/reactivex/SingleTransformer.java
index b4cf57b57e..bb0a7c1a06 100644
--- a/src/main/java/io/reactivex/SingleTransformer.java
+++ b/src/main/java/io/reactivex/SingleTransformer.java
@@ -13,6 +13,8 @@
 
 package io.reactivex;
 
+import io.reactivex.annotations.*;
+
 /**
  * Interface to compose Singles.
  *
@@ -26,5 +28,6 @@ public interface SingleTransformer<Upstream, Downstream> {
      * @param upstream the upstream Single instance
      * @return the transformed SingleSource instance
      */
-    SingleSource<Downstream> apply(Single<Upstream> upstream);
+    @NonNull
+    SingleSource<Downstream> apply(@NonNull Single<Upstream> upstream);
 }