diff --git a/google-cloud-datastore/src/main/java/com/google/cloud/datastore/Datastore.java b/google-cloud-datastore/src/main/java/com/google/cloud/datastore/Datastore.java
index 5bd8384a3..45cecbf25 100644
--- a/google-cloud-datastore/src/main/java/com/google/cloud/datastore/Datastore.java
+++ b/google-cloud-datastore/src/main/java/com/google/cloud/datastore/Datastore.java
@@ -51,6 +51,13 @@ public interface Datastore extends Service<DatastoreOptions>, DatastoreReaderWri
    * @param <T> the type of the return value
    */
   interface TransactionCallable<T> {
+    /**
+     * Callback's invoke method for the TransactionCallable.
+     *
+     * @param readerWriter DatastoreReaderWriter associated with the new transaction
+     * @return T The transaction result
+     * @throws Exception upon failure
+     */
     T run(DatastoreReaderWriter readerWriter) throws Exception;
   }