Some of the most prominent leaders of state throughout history have attempted to reshape the international system so that it can ensure peace and prosperity for the peoples of the world. Yet, despite their efforts, the system still falls short. Why is it that the number of international rules and institutions have grown exponentially over the past centuries, while some of the basic elements of relations between states remain basically unchanged? This book approaches the question through a theory of paradigmatic changes. By identifying the most basic principles of the international system, and how they endure or have been altered, it seeks to contribute to an improved understanding of possibilities and constraints in improving the system. The book discusses how a paradigmatic approach can explain the existing international rules, regimes and institutions in selected areas of the international system, including international peace and security, international trade, human rights, and international humanitarian and criminal law. Throughout, it considers the possibilities for reform of the system, and identifies possibilities (and impossibilities) for the future.