Virtually all academic books on American third parties in the last half-century assume that they have largely disappeared. This book challenges that orthodoxy by explaining the (temporary) decline of third parties, demonstrating through the latest evidence that they are enjoying a resurgence, and arguing that they are likely to once again play a significant role in American politics. The book is based on a wealth of data, including district-level results from US House of Representatives elections, state-level election laws after the Civil War, and recent district-level election results from Australia, Canada, India, and the United Kingdom.