One goal of developmental psychopathology is to understand the origins and course of mental disorders. I argue that pursuit of this goal requires a valid conceptual understanding of disorder and that this understanding can be provided by the “harmful dysfunction” analysis of the concept of disorder. The harmful dysfunction analysis holds that a disorder is a condition that is both harmful according to social values and caused by an internal dysfunction, that is, by a failure of an internal mechanism to perform a function for which it was naturally selected. This analysis explains why many of the distinctive features of developmental psychopathology are appropriate to the study of disorder. It is argued that the harmful dysfunction analysis is a necessary supplement to other proposed criteria for disorder, such as developmental deviation or predictive validity.