I was at DevWeek last week, it was a great week and I got a chance to speak with some of my geek heroes (Jeff Richter is a walking, talking CLR). One of the folks I most enjoyed listening to was ThoughtWorker Neal Ford who gave a session on emergeant design in TDD. Something struck me about the RGR cycle in TDD in that design could either be missed or misplaced if the refactor phase is never carried out and after the inital green phase the design is considered done. In TDD the emergant design that evolves as part of the cycle is key to the approach.  Neal talked about using cyclometric complexity as a measure of your emerging design but other considerations would surely include SOLID and DRY during the cycles. As you refactor to these kinds of design principles your design evolves.