Test-Driven Development (TDD) is a simple idea: write the tests for your code before writing the code itself. This "deceptively simple" reversal, however, fundamentally changes the role that testing plays in the development process and challenges assumptions about what testing is for.  Starting with an overview of TDD, Freeman and Pryce fill in the practical details that actually make it work in object-oriented systems. They also present an extended example that tries to strike a balance between being large enough to be realistic and small enough to fit in a book.