The literature devoted to political parties at the European level - also known as the ‘Europarties’ - frequently argues that due to the impact of the EU’s peculiar institutional environment and the need to adapt to it, Europarties have converged on some common organizational features. Building on neo-institutional theories of party change and adaptation, and especially on historical and sociological institutionalism, this thesis offers an alternative set of explanations for the organizational forms of the Europarties. In so doing, it brings into question both the extent of their organizational convergence and the primary focus of the literature on environmental factors, as well the limits of the few comparisons between Euroand national level party organizations that have been made thus far. To this end, the research focuses on the organization of the two largest Europarties - the European People’s Party (EPP) and the Party of European Socialists (PES) - and on how and why they differ from one another. Europarty organizational development is explained through an assessment of the impact of the organizational traditions of the individual member parties on their EU-level counterpart. Building on data gathered through in-depth interviews, archival research, and personal observations of party proceedings, as well as on the analysis of the formal documents, this thesis delineates and compares the organization and institutional rules of the EPP and the PES over the more than 30-year life-span of their existence, both at the formal and informal level. It also presents a thorough account of the EU legal framework providing for direct financing of Europarties and assesses its impact on their organization and activities. This thesis demonstrates the existence of important organizational differences between the EPP and the PES relating, among other elements, to their structure, the design of their decision-making process, membership policy and how party goals are specified. This claim stems also from the argument that organizational differences between EPP and PES member parties are transmitted to the EU level, thus causing a similar variation between the EPP and the PES themselves. Therefore, choices made by politicians on the EU transnational party scene are influenced by the ways in which they think about national party politics, despite the very different institutional environment of EU politics. In this way, the more we study the Europarties, the more we learn about national political parties. It is also apparent that these organizational choices have been relatively constant since the beginning of the EPP and the PES. Path dependency and historical institutionalism therefore prove to be especially powerful explanatory frameworks. In sum, the thesis has confirmed early assumptions that we need to look more deeply into organization of Europarties in order to gain more insight into their overall nature and role