In the last solo-authored edition of his seminal book, International Dispute Settlement, the late Professor John Merrills wrote that ‘the peaceful settlement of international disputes is the most critical issue of our time’.1 To the casual reader, this statement may seem an exaggeration. Surely the environment and climate change, collective security, global health, human rights, and international trade and finance, among others, may appear to be more pressing issues today. And yet, having devoted most of his professional life to the advancement of legal scholarship on international dispute settlement, Merrills was acutely aware that the study of this discipline offers the most effective litmus test to assess the strengths and limitations of international law at any given time. International disputes naturally arise whenever international law subjects disagree on matters of law or fact. Alongside disputes concerning territory and the use of force, today we witness an exponential growth of disputes on issues as diverse as human rights, environment, trade, and investment. Disputes are also becoming more complex, involving multiple parties, norms, and dispute settlement mechanisms (diplomatic or adjudicatory). At the same time, international disputes and the process of dispute settlement can act as a catalyst for the enforcement but also the development of international law. Dispute settlement and its institutions reaffirm existing norms and stabilise legal relations, but can also contribute to the crystallisation of new norms or new interpretations of existing norms because international disputes and their processes of settlement may evidence gaps or shortcomings of existing norms.