This dissertation aims at providing an answer to the question of how party system institutionalization occurs and why it varies so much across countries, through a comparative analysis of the process of party system institutionalization in East Central European democracies (i.e. Czech Republic, Hungary, Poland, and Slovakia). This work seeks to enhance the literature in three different ways. First of all, party and party system institutionalization are clearly considered as two different, although related, phenomena. Second, and building on Mair (1996, 2001), it attempts to advance party system analyses by improving the operationalization of institutionalization. In this context, a new operationalization which is both conceptually and empirically superior to the existing ones is suggested. Finally, and contrary to the tendency of explaining party system formation and development using either an institutional or a sociological approach, a serious effort is made to bring the two approaches together when examining the process of party system institutionalization in new democracies. The analysis here undertaken indicates that party system institutionalization in new East Central European democracies has been enhanced by both supportive institutional structures (i.e. political parties, electoral systems, type of regime) and strong “cleavage” structuration. The idea is that as individual political parties institutionalized (i.e. develop stable roots in society and solid organizations), they help voters to make the political expression of social cleavages more consistent, thereby avoiding instability in the patterns of inter-party competition for government. Second, party system format, itself a function of the type of electoral system adopted, also plays a role. The logic is that the lower the number of parties, the lower the transaction costs and the potential conflicts in terms of partisan interaction are likely to be. Third, in clear contrast to parliamentarian regimes, semi-presidentialism has a negative effect (both direct and indirect) on the process of party system institutionalization. Finally, the type of cleavage development is considered to have a positive (cumulative) or negative (cross-cutting) effect on the process of systemic institutionalization.