This thesis examines the impact of the changing dynamics of the international cooperation among communist (or post-communist) parties and Western social democratic parties on democratic transitions and on party change. This is done through an in-depth comparative study of the inter-party contacts between the communist and later post-communist parties of Poland and Hungary, on one hand, and the German SPD and the British Labour Party on the other. The thesis analyzes the scope of these contacts, the activity of bilateral groups, and the support offered to the Polish and Hungarian post-communist parties before and after 1989. The literature on democratization in post-communist Europe and that on post-communist parties in particular has neglected this issue, and the importance of inter-party contacts therefore tends to be overlooked. This study shows that the period prior to the system collapse in Central and Eastern Europe (CEE) was not a tabula rasa in terms of contacts between the parties from East and West. Relying on extensive interviews and unprecedented primary research in archival documents, it broadens the discussion, unearthing new material concerning the pre-1989 inter-party contacts and the reasons behind these contacts, as well as offering an original analysis of party goals and strategies of cooperation in the Cold War environment. It brings in international factors to offer a fuller explanation of the post-communist parties’ successful accommodation to the new reality, emphasising the importance of their antecedent socialization in the social democratic environment long before the collapse of the communist system. It points to the different trajectories of inter-party cooperation and the diverse policies pursued by parties in CEE and in Western Europe and explains these in terms of geographic proximity and their respective foreign and domestic policies. It further traces how personal links between individuals were maintained despite the communist regime collapse and party competition in post-1989 Hungary and Poland. More generally, this thesis emphasises the importance of taking these particular aspects of party activity and development into account within the context of the democratization research.