This book addresses some of the following questions: What is the relevance of legitimacy, in general and today? How does legitimacy compare nationally and internationally? What are the components of legitimacy at the international level? What are the limitations of international law when it comes to legitimacy? How does legitimacy change over time at the international level? How can the international system, and the international law that comes with it, be made more just and legitimate? The book is organized into six parts and twenty-three chapters. Part I sets the stage for the book. Part II unpacks the meaning and role of legitimacy in politics. Part III turns attention to legitimacy at the international level. Part IV focuses on how international legitimacy is constructed in international law. Part V addresses change and international legitimacy. Part VI adopts a point of view that is at the same time critical and constructive or, more precisely, reconstructive of international order.