The direct primary stands as one of the most significant and distinctive political reforms of the Progressive era in American history. In this book, the authors provide the most comprehensive treatment available on the topic and utilize new data on election outcomes, candidate backgrounds, incumbent performance and behavior, newspaper endorsements, and voters' preferences. They begin by studying whether primary elections have achieved the goals set by progressive reformers when they were first introduced over a century ago. They then evaluate the key roles these elections have played in the US electoral systems, such as injecting electoral competition into the regions that are dominated by one of the two major parties, helping select relatively qualified candidates for office, and, in some cases, holding incumbents accountable for their performance. They conclude with studying the degree to which primaries are responsible for the current, highly polarized environment. Anyone interested in US primary elections, US political history, or electoral institutions more generally should read this book.