Scholars have long debated why and when states comply with international law; one widely held view is that states do so out of a sense of moral obligation or a desire for legitimacy. This elegantly argued book by two noted law professors offers a simpler and more instrumental explanation: states agree to and follow international law only when it is in their national self-interest. Using elementary game theory, they build a framework that sees international law primarily as a tool for states seeking to solve "games" of cooperation. In their view, much of international law thus reflects a coincidence of interests rather than the embodiment of obligatory universal norms. The book has the virtues and liabilities of all simple rationalist theories. It neatly organizes a wide array of international rules and institutions and traces it all back to self-interested states. It also joins the effort to build bridges between the traditionally separate worlds of international law and international relations. But it leaves unexamined the deeper questions of how and why states--particularly modern democracies--define their interests the way they do.