Legitimacy is not something distinct from power; it is one of the vital sources of power. And if power shapes the nature and development of international orders, then the politics of legitimacy features prominently in the construction, maintenance, and dissolution of such orders. This article begins by exploring the concepts of power and legitimacy, their theoretical interconnection, and the impact that crises of legitimacy have on the maintenance of political power. It then takes an empirical turn, examining, however briefly, two sites in which the politics of legitimacy had a profound effect on the development of the modern international order. The first concerns the globalization of the system of sovereign states, a four-century long process of imperial extension, crisis, and fragmentation into successor states, a process in which struggles over individual rights played a key role. The second concerns the definition and distribution of special responsibilities for managing functional challenges among states. In orders characterized by formal sovereign equality, on the one hand, and imbalances of material capabilities, on the other, a mechanism needs to be found for containing and harnessing power to meet the problems faced by the international community. Historically, this has been achieved through the allocation of special responsibilities to particular states, usually great powers. But if regimes of special responsibilities create patterns of more or less formal hierarchy among states, they are also sites of intense battles over legitimacy.