This report offers an analytical overview of recent scholarship on the effects of the primary election on politics and the effects of different primary rules on voters, candidates, and policy moderation. Though many studies have been conducted in recent years, this is the first time that they have been systematically brought together with the express purpose of drawing comprehensive lessons. While primary elections are likely contributors to hyper-partisanship, their impact is more marginal than many popular accounts suggest. Yet research does show that primaries incentivize more polarizing behavior among candidates and legislators.