My opinion is that TDD can only reasonably be done if you already know what your implementation must be, which is difficult to do unless * you have already don

My opinion is that TDD can only reasonably be done if you already know what your implementation must be, which is difficult to do unless * you have already done it, or something sufficiently similar, before; * you have formal functional requirements; or * you have detailed use cases. Personally, I've recently come to really favor use cases as part of the design process. The mindset behind them requires some effort to learn, but I find them an effective vehicle for categorically separating what an application must do from any consideration for how it might do it.