Concerns about the position and function of nation-states in the international arena have led to a growing interest in the role of cities in international relations. This timely book advances the argument that cities are becoming active and informal actors in international law-making, indicating the emergence of a 'third generation' of multi-level governance. Expansive in scope, the book investigates various areas of city cooperation such as the economy, migration, security, sustainable development, ecology, and the position of cities in international law. Interviews conducted with the official representatives of several cities and international institutions, including UN-Habitat, the EU Committee of the Regions, and the Congress for Local and Regional Authorities of the Council of Europe, offer key insights into the most pressing urban issues of the 21st century. Examining the latest information on the international activities of cities, this engaging book explores the possibility that cities may soon reach the level of international subjects, capable of both implementing and creating international law. Contributing to the under-represented literature on the evolving function of cities in the modern world, this prescient book will be of interest to academics and students of urban studies, international relations, political science, and international law. City authorities dealing with international cooperation will benefit from its consideration of further development opportunities.