Definitely, whenever I have a clearly defined design to implement. If I don't, I generally try to design the API/interface first, then write at least the basic

Definitely, whenever I have a clearly defined design to implement. If I don't, I generally try to design the API/interface first, then write at least the basic tests before implementing. That being said, I never write all the possible tests before starting with the implementation. They're called unit tests for a reason -- I generally write at least a few tests for a particular unit (say, a function or method) and then write the implementation before starting working on another. And I often go back and add extra tests for an already implemented unit to cover some edge cases and error conditions.