Chapter 4 tests the effects of entry costs and decentralized rules on membership. After describing overall membership trends, the chapter tests the effects of party decentralization on both aggregate membership and individual enrollment. In line with the model’s predictions, the empirical analyses reveal that voters are more likely to join decentralized parties, but that this relationship is driven by the preferences of extreme voters. The chapter also examines the effects of membership fees on enrollment and employs two member surveys from Canada to investigate how individuals’ reported reasons for joining relate to their participation in various party activities.