This study describes and compares Czech and Slovak party politics after 1989. The concept of cleavages is used as a theoretical starting point. The authors point out that although the communist period overshadowed the traditional cleavages dating from the second half of the nineteenth century, it is possible to analyze some politically-based cleavages in the respective party arrangements of the two countries. The main conclusion of the article is as follows: that despite differing trajectories of political development during the 1990s, at the present time, both the Czech and Slovak party systems show great similarities in terms of the prevalence of the socioeconomic cleavage. Socioeconomic cleavage emerged quite early after 1989 in the Czech Republic; in Slovakia the socioeconomic cleavage has become dominant only in recent years. This has contributed to the stabilization of the classic left-right model of political competition and the consolidation of the two countries' party systems.