diff --git a/core/java/android/os/UpdateEngine.java b/core/java/android/os/UpdateEngine.java
index bf03cce947e2a..8549cff1faad0 100644
--- a/core/java/android/os/UpdateEngine.java
+++ b/core/java/android/os/UpdateEngine.java
@@ -26,7 +26,18 @@ import android.util.Log;
 /**
  * UpdateEngine handles calls to the update engine which takes care of A/B OTA
  * updates. It wraps up the update engine Binder APIs and exposes them as
- * SystemApis, which will be called by system apps like GmsCore.
+ * SystemApis, which will be called by the system app responsible for OTAs.
+ * On a Google device, this will be GmsCore.
+ *
+ * The minimal flow is:
+ * <ol>
+ * <li>Create a new UpdateEngine instance.
+ * <li>Call {@link #bind}, optionally providing callbacks.
+ * <li>Call {@link #applyPayload}.
+ * </ol>
+ *
+ * In addition, methods are provided to {@link #cancel} or
+ * {@link #suspend}/{@link #resume} application of an update.
  *
  * The APIs defined in this class and UpdateEngineCallback class must be in
  * sync with the ones in
@@ -80,12 +91,20 @@ public class UpdateEngine {
 
     private IUpdateEngine mUpdateEngine;
 
+    /**
+     * Creates a new instance.
+     */
     @SystemApi
     public UpdateEngine() {
         mUpdateEngine = IUpdateEngine.Stub.asInterface(
                 ServiceManager.getService(UPDATE_ENGINE_SERVICE));
     }
 
+    /**
+     * Prepares this instance for use. The callback will be notified on any
+     * status change, and when the update completes. A handler can be supplied
+     * to control which thread runs the callback, or null.
+     */
     @SystemApi
     public boolean bind(final UpdateEngineCallback callback, final Handler handler) {
         IUpdateEngineCallback updateEngineCallback = new IUpdateEngineCallback.Stub() {
@@ -125,11 +144,42 @@ public class UpdateEngine {
         }
     }
 
+    /**
+     * Equivalent to {@code bind(callback, null)}.
+     */
     @SystemApi
     public boolean bind(final UpdateEngineCallback callback) {
         return bind(callback, null);
     }
 
+    /**
+     * Applies the payload found at the given {@code url}. For non-streaming
+     * updates, the URL can be a local file using the {@code file://} scheme.
+     *
+     * <p>The {@code offset} and {@code size} parameters specify the location
+     * of the payload within the file represented by the URL. This is useful
+     * if the downloadable package at the URL contains more than just the
+     * update_engine payload (such as extra metadata). This is true for
+     * Google's OTA system, where the URL points to a zip file in which the
+     * payload is stored uncompressed within the zip file alongside other
+     * data.
+     *
+     * <p>The {@code headerKeyValuePairs} parameter is used to pass metadata
+     * to update_engine. In Google's implementation, this is stored as
+     * {@code payload_properties.txt} in the zip file. It's generated by the
+     * script {@code system/update_engine/scripts/brillo_update_payload}.
+     * The complete list of keys and their documentation is in
+     * {@code system/update_engine/common/constants.cc}, but an example
+     * might be:
+     * <pre>
+     * String[] pairs = {
+     *   "FILE_HASH=lURPCIkIAjtMOyB/EjQcl8zDzqtD6Ta3tJef6G/+z2k=",
+     *   "FILE_SIZE=871903868",
+     *   "METADATA_HASH=tBvj43QOB0Jn++JojcpVdbRLz0qdAuL+uTkSy7hokaw=",
+     *   "METADATA_SIZE=70604"
+     * };
+     * </pre>
+     */
     @SystemApi
     public void applyPayload(String url, long offset, long size, String[] headerKeyValuePairs) {
         try {
@@ -139,6 +189,15 @@ public class UpdateEngine {
         }
     }
 
+    /**
+     * Permanently cancels an in-progress update.
+     *
+     * <p>See {@link #resetStatus} to undo a finshed update (only available
+     * before the updated system has been rebooted).
+     *
+     * <p>See {@link #suspend} for a way to temporarily stop an in-progress
+     * update with the ability to resume it later.
+     */
     @SystemApi
     public void cancel() {
         try {
@@ -148,6 +207,10 @@ public class UpdateEngine {
         }
     }
 
+    /**
+     * Suspends an in-progress update. This can be undone by calling
+     * {@link #resume}.
+     */
     @SystemApi
     public void suspend() {
         try {
@@ -157,6 +220,9 @@ public class UpdateEngine {
         }
     }
 
+    /**
+     * Resumes a suspended update.
+     */
     @SystemApi
     public void resume() {
         try {
@@ -166,6 +232,15 @@ public class UpdateEngine {
         }
     }
 
+    /**
+     * Resets the bootable flag on the non-current partition and all internal
+     * update_engine state. This can be used after an unwanted payload has been
+     * successfully applied and the device has not yet been rebooted to signal
+     * that we no longer want to boot into that updated system. After this call
+     * completes, update_engine will no longer report
+     * {@code UPDATED_NEED_REBOOT}, so your callback can remove any outstanding
+     * notification that rebooting into the new system is possible.
+     */
     @SystemApi
     public void resetStatus() {
         try {
diff --git a/core/java/android/os/UpdateEngineCallback.java b/core/java/android/os/UpdateEngineCallback.java
index b3b856fcf142b..afff60abb22bb 100644
--- a/core/java/android/os/UpdateEngineCallback.java
+++ b/core/java/android/os/UpdateEngineCallback.java
@@ -19,7 +19,8 @@ package android.os;
 import android.annotation.SystemApi;
 
 /**
- * Callback function for UpdateEngine.
+ * Callback function for UpdateEngine. Used to keep the caller up to date
+ * with progress, so the UI (if any) can be updated.
  *
  * The APIs defined in this class and UpdateEngine class must be in sync with
  * the ones in
@@ -31,9 +32,19 @@ import android.annotation.SystemApi;
 @SystemApi
 public abstract class UpdateEngineCallback {
 
+    /**
+     * Invoked when anything changes. The value of {@code status} will
+     * be one of the values from {@link UpdateEngine.UpdateStatusConstants},
+     * and {@code percent} will be valid [TODO: in which cases?].
+     */
     @SystemApi
     public abstract void onStatusUpdate(int status, float percent);
 
+    /**
+     * Invoked when the payload has been applied, whether successfully or
+     * unsuccessfully. The value of {@code errorCode} will be one of the
+     * values from {@link UpdateEngine.ErrorCodeConstants}.
+     */
     @SystemApi
     public abstract void onPayloadApplicationComplete(int errorCode);
 }