This book is about legitimacy in global governance - that is, how people, from citizens to elites, perceive of institutions such as the United Nations, International Monetary Fund, and European Union. The book argues that legitimacy is central for global governance institutions to effectively combat problems such as climate change, trade protectionism, and human rights abuses - but also increasingly challenged by populist politicians, emerging powers, and civil society organizations. Moving beyond limitations in previous scholarship, the book develops an agenda for systematic and comparative research on legitimacy in global governance. Substantively, the book covers three prominent themes: sources of legitimacy for global governance institutions; processes of legitimation and delegitimation in global governance; and consequences of legitimacy for problem-solving effectiveness in global governance.