JRuby is regarded as the most stable and efficient implementation of Ruby. Running JRuby on top of Google App Engine for Java gives access to many interesting possibilities, including running Rails on top of Google’s infrastructure. Exactly how to make this happen is not totally apparent, though, and there are several gotchas involved in the process. This presentation will introduce the pieces of JRuby needed to get everything working, what kind of customizations are needed, what you might need to avoid in Ruby to avoid getting in trouble with App Engine’s security restrictions, and many other things. It will also look at how to test your Rails application, and how you can interface Ruby with Google’s Datastore service. And JRuby isn’t the only dynamic language that can be run on top of App Engine, using the new Java stack. Ioke is a small, prototype based object oriented language that also runs on App Engine. This part of the presentation introduces the things that you as a language implementor might need to keep in mind to make sure that your JVM language can run on top of App Engine.