@chrisoldwood @tdpauw @samir @mrksdck Strange. I read it as a instruction manual for outside-in TDD: Start on the outside, identify the next level in, write the unit tests while mock anything deeper (it likely hasn't been written yet), repeat until the feature is complete (as defined by an external acceptance test). Fill out edge cases as appropriate (don't try to test all these from the outside). But granted it has been a while since I read it fully (erm…probably c.2010…).