Globalisation has created powerful new incentives for states to cooperate and has generated renewed interest in multilateralism, particularly in the European Union which has embarked on more extensive and intensive multilateralism. This book develops a modern definition of multilateralism and demonstrates how it can be understood and conceptualised in the 21st century. Tracing the evolution of the concept and practice of multilateralism, it examines its multiple forms, models and cases, and confronts debates about how it can be made durable and effective. The authors focus on the EU and assess how, and how effectively, the EU delivers on its commitment to effective multilateralism, globally and in its neighbourhood. They test and compare the EU experience in different policy areas, and include case studies on trade, foreign and security policy, market policies, conflict resolution, development, energy, migration, the UN, the Middle East and China. Shedding new light on 21st century multilateralism, this important book will be of interest to students and scholars of international relations, international organizations, European Union politics and foreign policy.