The post-1815 Concert of Europe was a watershed in diplomatic history, fostering a tradition of multilateral diplomacy. By the end of the nineteenth century, traders, jurists, professional groups, and social activists were working across borders to establish multilateral institutions to coordinate their activities. Lavelle argues that the interwar period was also a surprisingly important era in the evolution of multilateral cooperation. The United States failed to join the League of Nations, but private networks of cooperation flourished. Bankers negotiated deals to stabilize financial flows, and the Rockefeller Foundation was the driving force in fostering international cooperation on public health. Lavelle shows that the post-1945 explosion of multilateralism was possible precisely because of trial and error in earlier decades. Chapters explore the founding and evolution of the United Nations, the Bretton Woods system, the decolonization movement, alliances, and public health and environmental cooperation. Lavelle’s most important contention is that multilateralism is less an idealistic aspiration than a pragmatic tool for managing economic and security interdependence. Multilateralism may be in retreat today, but it remains the best solution for the world’s increasingly complex problems.