Reflecting on 2016, it might seem that the national parties have little control over how the presidential nominations unfold and who becomes their presidential candidate. Yet the parties wield more influence than voters in determining who prevails at the National Conventions. Although the reforms of the late 1960s and 1970s gave rank-and-file party members a clear voice in the selection of presidential candidates, the parties retain influence through their ability to set the electoral rules. Despite this capability, party elites do not always fully understand the consequences of the rules and therefore often promote a system that undermines their goals. The Primary Rules illuminates the balance of power that the parties, states, and voters assert on the process. By utilizing an original, comprehensive data set that details the electoral rules each party employed in each state during every nomination from 1976 to 2016, Caitlin E. Jewitt uncovers the effects of the rules on the competitiveness of the nomination, the number of voters who participate, and the nomination outcomes. This reveals how the parties exert influence over their members and limit the impact of voters. The Primary Rules builds on prior analyses and extends work highlighting the role of the parties in the invisible primary stage, as it investigates the parties’ influence once the nominations begin. The Primary Rules provides readers with a clearer sense of what the rules are, how they have changed, their consequences, and practical guidance on how to modify the rules of the nomination system to achieve their desired outcomes in future elections.