Google App Engine is designed to help developers more easily create and manage scalable web applications. But what about testing? We typically write tests under the assumption that our development stack closely resembles our production stack, so what do we do when our target environment only lives in the cloud? In this talk, we will highlight the key differences between typical testing techniques and Google App Engine testing techniques. We will also present concrete strategies for testing against local and cloud-based implementations of App Engine services like the datastore and the task queue. Finally, we will explain how to use App Engine as a highly parallel test harness that runs existing test suites without modification.