Why do some international courts wield broad authority while others face pushback or fade into irrelevance? This book provides novel theoretical and empirical insights into this question. It offers a fresh, interdisciplinary perspective on the authority of international courts, drawing from law, political science and behavioural research.It develops, in Part I, a theoretical model and analytical framework for assessing these questions. The model explains how authority relationships between courts and their audiences – such as states, NGOs, individual applicants, and domestic courts – are formed, maintained, or contested. In Part II, the book applies this analytical framework to in-depth case studies of two selected international courts: the European Court of Human Rights and the Court of Justice of the Economic Community of West African States. These studies not only provide deeper insights into the authority of these courts but also demonstrate how the analytical framework can be applied to other international courts to generate an enhanced understanding of international court authority.The book sheds new light on why some courts enjoy enduring authority and others face pushback, offering powerful tools for understanding the behaviour of international courts and their audiences.