This article is part of the special cluster titled Parties and Democratic Linkage in Post-Communist Europe, guest edited by Lori Thorlakson, and will be published in the August 2018 issue of EEPSIn an article written in 1995 titled “What Is Different about Postcommunist Party Systems?” Peter Mair applied the method that he called “ex adverso extrapolation.” He matched his knowledge of the process of consolidation of party systems in the West with what was known at that time about Eastern European history, society, and the emerging post-communist party politics. Considering factors such as the existence of fluid social structures, the weakness of civil society, or the destabilizing impact of the so-called triple transition, his article predicted long-term instability for the region. In the present article, we evaluate the validity of Mair’s predictions, thereby also contributing to a lively debate in the current literature about the scale and nature of East–West differences and about the trajectories of the two regions. Going beyond the identification of cross-regional similarities and differences, we also differentiate between individual party systems, establish subgroups, and describe changes across time. Using four major dimensions (i.e., party system closure, party-level stability, electoral volatility, and fragmentation), the article finds that Mair’s predictions were largely, though not in every detail, right. Ironically, however, we also find that changes in the West tend to match over time the trajectory of the East.