This volume addresses the normative legitimacy of international institutions, asking how we can make sense of legitimacy claims of increasingly diverse global governance institutions and practices and how their legitimacy relates to and differs from state legitimacy. State legitimacy is a central concern of modern political thought but is inadequate when applied to institutions that differ from the state in type, level of governance, scope, and much else. We need a new, tailored approach to the legitimacy of institutions beyond the state, especially international and transnational institutions. Such an approach includes foundational questions such as, what does it mean for institutions to be legitimate that have radically different purposes, means, interests, capacities, constituents, and roles from states? And what standards do such institutions have to meet in order to count as legitimate? The contributions to this volume seek to advance the debate on these questions at both abstract and more concrete levels. They range from conceptual questions about the nature of legitimacy and international institutions, to rule of law, to the legitimacy of the UN Security Council, the International Criminal Court, and occupying military forces in the face of challenges specific to their nature and context. Together they demonstrate both the promise and challenges of theorizing legitimacy beyond the state. The chapters in this book were originally published as a special issue of the journal, Critical Review of International Social and Political Philosophy.