This book examines the representativeness of party membership and analyses the potential consequences of changing representativeness. Parties with high membership ratios, as well as those experiencing severe decline, are compared and examined across countries with varying constitutional arrangements and party systems. The book discusses whether changing representative capacities lead to declining political representation of (group) interests, less representative party candidate selection processes and declining legitimacy for the political system. The book bridges two subareas that are usually not in conversation with each other: literature on the decline of party membership and that on group representation (gender, ethnic minorities and other social groups). This text will be of key interest to students and scholars of party politics, political parties, representation and elections, and more broadly to people interested in European and comparative politics.