One of this morning’s sessions at Alt.NET 2010 discussed BDD. Charlie Pool expressed the opinion, which I have heard many times, that BDD is just a description of TDD done properly.

For me, the core principles of BDD are:

• expressing behaviour in terms that show the value to the system actors
• Expressing behaviours / scenarios in a format that clearly separates the context, the action and the observations.

If we go back to Kent Beck’s TDD book neither of these elements are mentioned as being core to TDD. BDD is an evolution of TDD. It is a specialisation of TDD, but it is not the same as TDD. Discussing BDD, and building specialised tools for BDD, is valuable even though the difference between BDD and TDD is subtle. Further, the existence of BDD does not mean that TDD is obsolete or invalidated.