This article provides a comprehensive theoretical model for analysing the influence of European party federations (Europarties) on their Central and Eastern European (CEE) partner parties. It draws on the concepts of Europeanization and party change and is, in principle, applicable to similar processes of party enlargement elsewhere. In the process of their Eastern enlargement, Europarties have searched for suitable CEE partner parties on which they have tried to exert influence. Thus far, little is known about the precise mechanisms involved and the actual impact of these processes. We argue that Europarty influence leads to party change on the side of the CEE partners, which brings about ‘West-Europeanization’. Europarties are expected to apply a double strategy that consists of a process of political exchange based on a largely asymmetric power relationship in favour of Europarties and a socialization strategy in which Europarties try to socialize their partners and convince them of the appropriateness of change. We present the theoretical and analytical tools for the analysis of the arguably largest extension of transnational party organizations, which provides ample testing ground for more general theories on party change. Furthermore, we present preliminary empirical evidence on the two most important cases, namely the European People’s Party and the Party of European Socialists, which indicates that Europarties played an important role in the development of CEE parties and party systems in general.