The success of design-oriented companies like Apple means that everyone understands the importance of interfaces. But what are these external indicators of success based on? What have we achieved, what do we know, and why are others interested in what we do? Where should we go from here? Here, I first outline three successes: transformative technology, the importance of experience, and the user-centric design process. I then argue that now we need to build on these successes by taking a more principled approach to what we do.