Global governance has a major impact on the lives of people around the world. However, traditional theories of legitimacy were usually developed for states and are not suitable for the diversity of global governance institutions that exist today. This book first develops a normative concept of legitimacy that is applicable to all political institutions. According to this concept, to regard an institution as legitimate means ascribing it the right to exercise its function in political practice. Secondly, the book discusses how the use of this concept opens up new perspectives in the debate on legitimacy criteria for global governance institutions. In this context, the book analyses the relationship of legitimacy to the values of justice and democracy and discusses the role of feasibility constraints and the all-affected principle in legitimacy judgements. The concept of legitimacy as the right to function opens up the conceptual space to accommodate the insight that legitimacy criteriaare not the same for all global governance institutions, but depend on their function and context. Thirdly, the book applies the developed theoretical framework to a specific global governance institution, the G20.