diff --git a/README.md b/README.md
index bcdb231c3..368cbf10f 100644
--- a/README.md
+++ b/README.md
@@ -216,7 +216,8 @@ See [TESTING] to read more about testing.
Versioning
----------
-This library follows [Semantic Versioning](http://semver.org/).
+This library follows [Semantic Versioning](http://semver.org/), but does update [Storage interface](src/main/java/com.google.cloud.storage/Storage.java)
+to introduce new methods which can break your implementations if you implement this interface for testing purposes.
It is currently in major version one (``1.y.z``), which means that the public API should be considered stable.
diff --git a/google-cloud-storage/src/main/java/com/google/cloud/storage/Storage.java b/google-cloud-storage/src/main/java/com/google/cloud/storage/Storage.java
index c95213d85..a9bb589a2 100644
--- a/google-cloud-storage/src/main/java/com/google/cloud/storage/Storage.java
+++ b/google-cloud-storage/src/main/java/com/google/cloud/storage/Storage.java
@@ -19,6 +19,7 @@
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
+import com.google.api.core.InternalExtensionOnly;
import com.google.api.gax.paging.Page;
import com.google.auth.ServiceAccountSigner;
import com.google.auth.ServiceAccountSigner.SigningException;
@@ -55,6 +56,7 @@
*
* @see Google Cloud Storage
*/
+@InternalExtensionOnly
public interface Storage extends Service {
enum PredefinedAcl {