A conference and the book it produces are made possible by many contributions of different kinds. To begin, there is the team that conceptualized and organized the whole enterprise. I owe a debt of gratitude to MSRI’s Educational Advisory Committee: Bruce Alberts, Michèle Artigue, Deborah Ball, Hyman Bass, David Eisenbud, Roger Howe, Jim Lewis, Robert Moses, Judith Ramaley, Hugo Rossi, and Lee Shulman. A planning team consisting of Deborah Ball, Hyman Bass, David Eisenbud, Jim Lewis, and Hugo Rossi put in many hours organizing and running the conference. David and Hugo were responsible for much of the conference infrastructure, which is typically invisible (when things go well, which they did) but absolutely essential. An equally essential part of the infrastructure is fiscal: I am grateful to the National Science Foundation, the Noyce Foundation, and the Spencer Foundation for their collective underwriting of the conference. Of course, the participants and the authors are who make the conference and the volume what they are. It has been a pleasure to work with them, individually and collectively. Most special thanks are due to Cathy Kessel, who worked with me in editing this volume. Every chapter—every word, every figure—has been reviewed by Cathy, and the manuscript is much improved for her efforts. Thanks as well to Silvio Levy for turning our drafts into technically viable LATEX, and to Cambridge University Press for converting Silvio’s labors into the volume you now hold in your hand.