The intervention of States in legal proceedings touches upon some of the most beguiling questions in international dispute settlement. These include questions of treaty interpretation, obligations erga omnes, the sources of judicial power and rulemaking, the nature of incidental proceedings, the Monetary Gold doctrine of indispensable parties, cross-fertilization between judicial and arbitral bodies, and principles of jurisdiction, party autonomy, and res judicata. As jurists and scholars tend to address these questions in isolation, however, each development in third-State practice has raised unimagined issues of first impression-such as the 2022 statement of dozens of States exploring mass intervention before the International Court of Justice in Ukraine v. Russia, and the participation of neighbouring States without China's presence in the 2016 South China Sea arbitration. By applying conceptual, comparative, and historical approaches to international justice, this book instead offers a uniquely holistic assessment of the practice and prospective development of intervention.