In the aftermath of war, what determines whether peace lasts or fighting resumes, and what can be done to foster durable peace? Drawing on theories of cooperation, I argue that belligerents can overcome the obstacles to peace by implementing measures that alter incentives, reduce uncertainty about intentions, and manage accidents. A counterargument suggests that agreements are epiphenomenal, merely reflecting the underlying probability of war resumption. I test hypotheses about the durability of peace using hazard analysis. Controlling for factors (including the decisiveness of victory, the cost of war, relative capabilities, and others) that affect the baseline prospects for peace, I find that stronger agreements enhance the durability of peace. In particular, measures such as the creation of demilitarized zones, explicit third-party guarantees, peacekeeping, and joint commissions for dispute resolution affect the duration of peace. Agreements are not merely scraps of paper; rather, their content matters in the construction of peace that lasts.Many friends and colleagues have given advice and comments on the larger project of which this paper is a part. In particular I would like to thank Scott Bennett, Nora Bensahel, Erik Bleich, Dan Drezner, Lynn Eden, Nisha Fazal, Jim Fearon, Wendy Franz, Erik Gartzke, Chris Gelpi, Doug Gibler, Hein Goemans, Amy Gurowitz, Lise Howard, Bob Jervis, Bob Keohane, Zeev Maoz, Lisa Martin, Dani Reiter, Don Rothchild, Evan Schofer, Curt Signorino, Jack Snyder, Al Stam, Celeste Wallander, Barb Walter, Suzanne Werner, and four anonymous reviewers. I am grateful also for research assistance from Carol St. Louis. This project would not have been possible without financial and intellectual support from the Olin Institute at Harvard University, the Center for International Security and Cooperation at Stanford University, and the Institute of War and Peace Studies at Columbia University.