This chapter examines the legitimacy of international law.It begins with an analysis of the concept of legitimacy, or perhaps more precisely, of legitimate authority.It then considers four possible grounds for international law’s legitimacy: enhancing its subjects’ ability to act as they have most reason to act, the consent of those it claims as subjects, considerations of fair play, and international law’s democratic credentials.The chapter concludes with an examination of reasons why we should care about international law’s legitimacy; indeed, why from a moral point of view increasing the international legal order’s legitimacy might even take priority over making it more just.