Memcache is the most powerful tool to minimize response time and Datastore cost of your App Engine application. In this lesson, we will learn what Memcache is and when to use it.  We will also review the Memcache API in both Java and Python and look into some special usage including Batch operation and Atomic operation. In the latter part of the lesson, we present some caveats to know and best practices to use Memcache effectively, including the volatility, non-transactional characteristic, and how Memcache entries can be evicted.