The main objective of this report is to assess the impact of institutional factors and elections on the transnationalization of European parties and on the evolution of the European Union's (EU) party system. Institutional factors, which are effective during the European Parliament's (EP) legislative term, appear to favor the consolidation of the longer-established and larger EP party groups (EPP, PES, LDR) and ultimately of the EU party system. By contrast, elections can be very disruptive, especially for the more recent and smaller EP groups, and are a negative factor in party system consolidation. The two combined effects appear to have contributed to the creation of a two-speed party system in the EU, characterized by an increasingly institutionalized core and by a mutable and unstable periphery.