Environmental problems like global climate change and stratospheric ozone depletion can only be remedied if states cooperate with one another. But sovereign states usually care only about their own interests. So states must somehow restructure the incentives to make cooperation pay. This is what treaties are meant to do. A few treaties, such as the Montreal Protocol on Substances that Deplete the Ozone Layer, succeed. Most, however, fail to alter the state behaviour appreciably. This book develops a theory that explains both the successes and the failures. In particular, the book explains when treaties are needed, why some work better than others, and how treaty design can be improved. The best treaties strategically manipulate the incentives states have to exploit the environment, and the theory developed in this book shows how treaties can do this. The theory integrates a number of disciplines, including economics, political science, international law, negotiation analysis, and game