THIS BOOK OWES its existence largely to the support and encouragement of a number of individuals and institutions. We are greatly indebted to feedback from Sarah Anzia, Michael Bailey, Adam Berinsky, John Brehm, Jake Bowers, Andrea Campbell, Jamie Druckman, Brian Gaines, Daniel Hopkins, James Kuklinski, Paul Lewis, Paul Peterson, Robert Putnam, Tom Rudolph, Betsy Sinclair, and Cara Wong. Earlier versions of this work were presented at Harvard, Georgetown, and the University of Chicago, and we are grateful to these institutions for providing support of this work. We are especially grateful to Chris Berry, Elizabeth Gerber, William Howell, and Jessica Trounstine, who were incredibly generous in sharing data, time, and attention to this project. Matt Muttino and Ben Oren also provided outstanding research assistance. This research was financially supported by a Young Investigators CAREER grant from the National Science Foundation and by additional research support from the University of Chicago.