We had the benefit of comments from many friends and colleagues as this project progressed. Thanks are due to Alberto Abadie, Patrick Arni, David Autor, Amitabh Chandra, Monica Chen, Victor Chernozhukov, John DiNardo, Peter Dolton, Joe Doyle, Jerry Hausman, Andrea Ichino, Guido Imbens, Adriana Kugler, Rafael Lalive, Alan Manning, Whitney Newey, Derek Neal, Barbara Petrongolo, James Robinson, Gary Solon, Tavneet Suri, Jeff Wooldridge, and Jean-Philippe Wullrich, who reacted to the manuscript at various stages. They are not to blame for our presumptuousness or remaining mistakes. Thanks also go to our students at LSE and MIT, who saw the material first and helped us decide what's important. We would especially like to acknowledge the skilled and tireless research assistance of Bruno Ferman, Brigham Frandsen, Cynthia Kinnan, and Chris Smith. We're deeply indebted to our infinitely patient illustrator, Karen Norberg, who created the images at the beginning of each chapter and provided valuable feedback on matters large and small. We're also grateful for the enthusiasm and guidance of Tim Sullivan and Seth Ditchik, our editors at Princeton University Press, and for the careful work of our copy editor, Marjorie Pannell, and production editor, Leslie Grundfest. Last, but certainly not least, we thank our wives for their love and support. They know better than anyone what it means to be an empiricist's companion.