Legitimacy is central for the capacity of global governance institutions to address problems such as climate change, trade protectionism, and human rights abuses. However, despite legitimacy’s importance for global governance, its workings remain poorly understood. That is the core concern of this volume, which engages with the overarching question: whether, why, how, and with what consequences global governance institutions gain, sustain, and lose legitimacy. This introductory chapter explains the rationale of the book, introduces its conceptual framework, reviews existing literature, and presents the key themes of the volume. It emphasizes in particular the volume’s sociological approach to legitimacy in global governance, its comparative scope, and its comprehensive treatment of the topic. Moreover, a specific effort is made to explain how each chapter moves beyond existing research in exploring the book’s three themes: (1) sources of legitimacy, (2) processes of legitimation and delegitimation, and (3) consequences of legitimacy.