Market design is a team sport, so I owe a great debt to all those who have worked on the markets reported here, many of whom are named in the course of telling the story. It turns out that making a book is also more of a team sport than I imagined. I received lots of help on this book, and would have received more if I were easier to help. Worth special mention are my agent, Jim Levine; Tim Gray, who conducted interviews of participants in kidney exchange and school choice; Mike Malone, who made my paragraphs shorter and clearer (and who knows a lot about Sooners); and my publisher, Eamon Dolan, who had clear ideas about what should be included in the book and what should not. I’m also indebted to Barbara Jatkola for thoughtful copyediting, and to Atila Abdulkadiroğlu, Eric Budish, Neil Dorosin, Alexandru Nichifor, and Parag Pathak for careful reading and insightful comments.