Test-Driven Development, or TDD, is often quoted as an essential Agile best practice, and so it is. It works wonders on green-fields projects and new code bases where you can start afresh and ensure that all your code is both easily testable and well tested. But what about legacy code? (By legacy code, I mean any code that does not have a comprehensive set of automated tests, so you might be writing legacy code as we speak). For most of us, most of the code we will ever work on will not have originally been our own work. And, unfortunately for the software industry, only a small fraction of code can really boast comprehensive unit and integration tests. How can techniques like Test-Driven Development make our work as developers more productive and less frustrating?