Robotium is an open source testing framework for Android applications. It makes it easy to write complex black-box test cases. Test driven development is a methology where tests are written before the actual application.
Write a testcase
Watch it failing
Write the actual source code in you application which enables the test to pass
Watch the test passing
Go back to step 1
The following video gives you an introduction into Robotium:
I will definitely use Robotium for my future Android projects. It is really worth having a look at it.
There is another interersting video: