This chapter examines the concept of an authoritarian regime. Aside from the fact that they are not democracies, authoritarian regimes have little in common and are considerably diverse: from monarchies to military regimes, from clergy-dominated regimes to communist regimes, and from seeking a totalitarian control of thought through indoctrination to seeking recognition as a multiparty democracy through using semi-competitive elections. The chapter first traces the historical evolution of authoritarian regimes, with particular emphasis on the three-phase modernization of dictatorship in the nineteenth and twentieth centuries. It then explores the key questions of who rules an authoritarian regime, why they rule (their claim to legitimacy), and how they rule (their mechanisms of control). Finally, it considers two differing perspectives on the past and future of authoritarianism: the extinction interpretation and the evolution interpretation.