Psychology has been reforming its own practices over the last decade. Most of that reform has focused on experiments and statistics while neglecting theory building. The history of cognitive science and mathematical psychology demonstrates the power of formal theory building. However, adoption of a formal approach in many areas of psychology seems to have been hindered by unfamiliarity. Much of psychology is still primarily in the business of discovering and confirming effects. This leaves contemporary psychology without strong formal and computational foundations, a scarcity of theoretical rigor, and a proliferation of just-so stories. In this talk, I will discuss how psychological science can improve its theoretical foundations by picking up tools for theory development.