In this chapter, we examine the ‘why’ question from the individual perspective. We analyse motivations for enrolment as expressed by party members themselves. First, we briefly discuss likely reasons for joining a political organization as suggested in the literature. We base our analysis on a general typology, looking at political, material, and social motivations for party membership. In the second part, we look at some background factors that may influence the motivation to join, specifically, the age when enrolling, member cohort, and former party membership. Third, we analyse members’ motivations to see if these differ among countries, party families, and membership generations. Fourth, we look more closely at the extent to which the social milieu, in particular family, matters for enrolment. We find that members are motivated to join primarily out of political interest. Ideology and policy interests link civil society and parties through their members. Moreover, social networks and family ties are important catalyzers of membership. Finally, motivations indicate bottom-up recruitment interests rather than careerist motivations.