The legitimacy of international and regional organizations and their actions is frequently asserted and challenged by states and commentators alike. Their authorisations or conduct of military interventions, their structures of decision-making, and their involvement into what states deem to be domestic matters have all raised questions of legitimacy. As international organizations lack the coercive powers of states, legitimacy is also considered central to theirability to attain compliance with their decisions. Despite the prominence of legitimacy talk around international organizations, little attention has been paid to the practices and processes through which such organizations and their member states justify the authority these organizations exercise – how they legitimise themselves both vis-vis their own members and external audiences. This book addresses this gap by comparing and evaluating the legitimation practices of a range of international and regional organizations. It examines thepractices through which such organizations justify and communicate their legitimacy claims, and how these practices differ between organizations. In exploring the specific legitimation practices of international organizations, this book analyses the extent to which such practices are shaped by thestructure of the different organizations, by the distinct normative environments within which they operate, and by the character of the audiences of their legitimacy claims. It also considers the implications of this analysis for global and regional governance.