Beginning with an explanation of what treaties are, the book explores how they are made, how they are interpreted, and how they are implemented both internationally and within national legal systems. In doing this, Gardiner provides an account of the main features of the law of treaties and describes how treaties contribute to the formation and codification of international law. He also outlines how treaties are the foundation of international organizations which, in turn, through their constitutive treaties and those treaties which they develop, make extensive contributions to international law. The book concludes by looking at the role of treaties across multiple areas of international law, including human rights, trade and investment law, air and space law, telecommunications, international criminal law, and environmental law. Thus, the book shows that treaties, and the legal regimes which they create, now constitute the pre-eminent component of international law.