This book has benefited from comments from many friends and colleagues, including Curtis Bradley, Rachel Brewster, Einer Elhauge, Ryan Goodman, Derek Jinks, Ehud Kamar, Daryl Levinson, Jide Nzelibe, Kal Raustiala, Paul Stephan, Lior Strahilevitz, Alan Sykes, Eric Talley, Ben Wittes, Tim Wu, John Yoo, and numerous anonymous readers. We also thank participants at workshops at the University of Chicago, the University of Southern California, Yale, and Harvard. We received helpful research assistance from Nicole Eitmann, Brian Fletcher, Wayne Hsiung, Brian Killian, Bill Martin, Michael Vermylen, and Lora Viola. Many other people, too numerous to mention, commented on drafts of articles that were subsequently incorporated in revised form into this book. These earlier articles are “A Theory of Customary International Law,” 66 University of Chicago Law Review 1113 (1999); “Understanding the Resemblance between Modern and Traditional Customary International Law,” 40 Virginia Journal of International Law 639 (2000); “Sovereignty, International Relations Theory, and International Law,” 52 Stanford Law Review 959 (2000); “Moral and Legal Rhetoric in International Relations: A Rational Choice Perspective,” 31 Journal of Legal Studies S115 (2002); “Liberal Democracy and Cosmopolitan Duty,” 55 Stanford Law Review 1667 (2003); “Do States Have a Moral Obligation to Comply with International Law?” 55 Stanford Law Review 1901 (2003); and “International Agreements: A Rational Choice Approach,” 44 Virginia Journal of International Law 113 (2003). These articles have been shortened for the book and have been revised in response to criticism and in light of the evolution of our thinking about international law. In addition, the book contains much new material.