Established in the wake of the First World War, the League of Nations fundamentally transformed international politics, global governance and multilateral cooperation in a multitude of fields from the economy, labour and social affairs to colonial, minority and security questions. This Handbook analyses the central role of law in the construction of a new international order under the League of Nations. Drawing from innovative research of recent years that analyses the League of Nations through the prism of ultimate success and failure, it offers twenty-one rich chapters that showcase an interdisciplinary, contextual and archive-based approach with brand new and unexplored case studies that address key topics of the legal history of the League, the International Labour Organization and the Permanent Court of International Justice. Finally, it offers a new historical synthesis of how to understand the role of international law in international organizations during the interwar period.