Legitimacy is not something distinct from power|it is a vital source of power. And since power politics plays a key role in the nature and development of international orders, the politics of legitimacy can be expected to have featured prominently in the construction, maintenance, and collapse of such orders. This article begins by exploring the concepts of power and legitimacy, their theoretical and practical interconnection, and the impact that the politics of legitimacy has on the constitution and distribution of political power. For many international relations scholars, power flows directly from material capabilities. Yet history is full of examples where actors with considerable material capabilities have not been able to realize their interests or control political outcomes. There is much more to power, therefore, than meets the eye. As Max Weber understood it, all political power rests on perceptions of legitimacy as much as material capability. Indeed, power that rests on nothing more than material capabilities is inherently unstable. Power thus has multiple sources, material and non-material, and legitimacy is as essential to power as guns and money. The key thing to note, however, is that legitimacy is an inherently social phenomenon|an actor's identity or behavior isonly legitimate when judged so by others. Furthermore, while material capabilities are clearly important sources of political power, they do not, on their own, generate legitimacy. Others can judge a materially powerful actor in a variety of ways: they can judge it and its actions to be legitimate, or they can see it as threatening. Material capabilities have no inherent meanings. For a materially strong actor to be judged legitimate, its identity and actions have to be seen as consistent with social norms and values that others consider important.  Having set out this theoretical and conceptual framework, the article takes an empirical turn, examining two ways in which the politics of legitimacy has affected the nature and development of the modern international order. I define international orders assystemic configurations of political authority , as historically contingent, normatively sanctioned ways of distributing legitimate political power. In some international orders, like the present one, political authority is organized according to the principle of sovereignty: the system is divided into multiple territorially demarcated units of centralized authority. In other orders, legitimate power has been distributed according to different principles: heteronomy, empire, suzerainty, etc. Europe's medieval order is an example of heteronomy, the classic Chinese world order was a case of suzerainty.  I am interested here in two institutional characteristics of the present international order and how they have been shaped by the politics of legitimacy. The first concerns its global nature, the fact that for the first time in world history the sovereign state is the sole legitimate form of political organization, and virtually the entire surface of the globe is divided into the patchwork of such states. Sovereignty has become the universal organizing principle, although this is a strikingly new development, realized fully only in the last 50 years. The second concerns one of the fundamental institutional practices of the contemporary order: the definition and allocation of special responsibilities to particular sovereign states. All international orders develop fundamental institutions to facilitate coexistence and collaboration between political units. Hedley Bull famously listed the institutions of the modern international order as international law, diplomacy, the balance of power, war, and management by the great powers. It is the last of these that interests me here, as such management is one expression of the notion that in a world of legally equal sovereigns some actors can, and should, be allocated differential responsibilities for the provision of certain international goods. If my first concern relates to the universalization of sovereign equality, the second concerns the institutionalization of hierarchy within such equality.  The politics of legitimacy has been central to the development of both of these features of the modern international order. Until the second half of the twentieth century, the conjoined principles of sovereignty and empire structured political life on the globe. Over the course of five centuries, individual empires fragmented into successor sovereign states, and after 1945 the institution of empire itself collapsed. Crises of institutional legitimacy drove each of these processes crises that occurred when imperial hierarchy was challenged by the emergence and mobilization of new ideas about individual rights. The resulting universalization of sovereignty has produced a legally equalitarian international order, in which all sovereign units have the same legal standing and the same array of legal entitlements. Yet as this order has developed, more or less formal types of hierarchy have also emerged, the most notable being the distribution of special responsibilities to particular actors, principally great powers. Not surprisingly, the definition and allocation of such responsibilities has been a focal point for the politics of legitimacy. Special responsibilities are legitimate social powers: they are rights or obligations to exercise a state's capabilities, or refrain from doing so, in the service of international social ends: order, economic well-being, humanitarianism, environmental protection, etc. In the complex international order, however, this politics is immensely complex, as special responsibilities are defined and distributed differently in different issue-areas.