Wikipedia: Test-driven development (TDD) is a software development process relying on software requirements being converted to test cases before software is fully developed, and tracking all software development by repeatedly testing the software against all test cases. This is opposed to software being developed first and test cases created later. Once you get into it, TDD is a beautifully satisfying experience: we write the specification of software to be built in the form of programmatic tests; these tests will all initially fail (turn red).