Résumé : Since the early 1990s, increasing academic attention has been devoted to party membership. Numerous studies have evidenced a long-term declining trend affecting almost all traditional parties in Western Europe (Mair and Van Biezen, 2001; Van Biezen et al., 2012b). Yet, in recent years, there have been some signs of a revival of party membership (Whiteley et al., 2019). What are the main factors accounting for fluctuations in party membership levels across Western Europe from the 1990s until 2014? This is the main question this dissertation seeks to answer. The main objective of this dissertation is to identify the factors that significantly affect the ability of political parties to recruit members in Western Europe. So far, the academic literature has mainly focused on micro- and macro-level determinants of membership fluctuations and have involved long-term explanations of shifts in party membership. Their general focus has been to ask why citizens join political and not so much why and in which conditions political parties are able to recruit members. The impact of meso-level and short-term factors on party membership variations has been largely underexplored. To shed new light on these issues, this study proposes to apply theoretical perspectives and empirical tools developed by sociological and economical organization studies. Four main theoretical perspectives have been developed by organizational theories to explain variations in organizations’ size and structure: the evolutionary system perspective (ES), the sociological neo-institutionalism (SI), transaction cost theory (TCT) and the resource-based view (RBV). Explanatory insights from each of these perspectives were identified and explored in each of the four empirical chapters of this dissertation. Overall, this dissertation evidences several transformations in party membership. By diversifying temporal perspectives, units of analysis and levels of observation, it shows that the decline of party membership levels is not as universal and as linear as it is often assumed. Membership levels are affected by electoral and organizational lifecycles. Not all parties have been affected by the general decrease in membership levels and some new parties have managed to attract an increasing number of members. Besides, parties that have given their members a greater say in their internal decision making have generally managed to attract new members. By looking at infra-national dynamics of party membership, this dissertation also shows the importance of regional and local context and the heterogeneity of membership trajectories within the same party. It underlines the importance of electoral mobilization at the local level and the importance of individual recruiters for the composition of the membership. By reflecting on the causes of party membership fluctuations, this dissertation sheds light on some important challenges for the future of our representative democracies.