Underlying any system that does design is a model of the design process and a division of labor between the system and the user. We are just beginning to understand what the main alternative models are, what their strengths and weaknesses are, and for which domains and tasks each is appropriate. The research reported here is an attempt to further that understanding by studying a particular model, the model of design as top down refinement plus constraint propagation, with the user making control decisions and the system carrying them out. We have studied this model by embodying it in VEXED, a design aid for NMOS digital circuits, and by experimenting with this system. Our primary conclusion is that this model needs further elaboration, but seems like a good basic model on which to build such systems.