This article provides an introduction to the basic institutional features of constitutional courts (CCs), as well as an overview of the small but growing comparative literature on their design, function, impact, and legitimacy. It presents the CC as an ideal type, with its own functional logics, and surveys the comparative scholarship seeking to explain commonalities and differences across systems. It emphasizes inter-disciplinarity, in part, because political scientists have been at the forefront of empirical research and, in part, because powerful CCs have shaped and reshaped their own political environments. Successful CCs routinely subvert separation of powers schemes, including elements on which their legitimacy was originally founded. In consequence, new legitimacy questions and discourses have emerged.