While extensive research examines electoral systems and institutions at the country-level, few studies investigate rules within parties. Inside Parties changes the research landscape by systematically examining 65 parties in 20 parliamentary democracies around the world. Georgia Kernell develops a formal model of party membership and tests the hypotheses using cross-national surveys, member studies, experiments, and computer simulations of projected vote shares. She finds that a party's level of decentralization – the degree to which it incorporates rank and file members into decision making – determines which voters it best represents. Decentralized parties may attract more members to campaign for the party, but they do so at the cost of adopting more extreme positions that pull them away from moderate voters. Novel and comprehensive, Inside Parties is an indispensable study of how parties select candidates, nominate leaders, and set policy goals.