Anxiety, fear, and sadness are a normal—even adaptive—part of the human emotional experience. However, the same emotions form the core of anxiety and depressive disorders that are among the leading causes of disability worldwide. What distinguishes clinical anxiety and depression from normal emotional experiences? Why do anxiety and depression so often co-occur? The answers to these questions have far-reaching implications for the science and practice of clinical psychology. The search for causes of emotional disorders, the development of targeted treatments, the accurate diagnosis of those in need of services, all depend on “carving nature at its joints.” Despite their impact, however, these are questions on which clinical scientists still strongly disagree.