How cohesive are political parties in the European Parliament? What coalitions form and why? The answers to these questions are central for understanding the impact of the European Parliament on European Union policies. These questions are also central in the study of legislative behaviour in general. We collected the total population of roll-call votes in the European Parliament, from the first elections in 1979 to the end of 2001 (over 11,500 votes). The data show growing party cohesion despite growing internal national and ideological diversity within the European party groups. We also find that the distance between parties on the left-right dimension is the strongest predictor of coalition patterns. We conclude that increased power of the European Parliament has meant increased power for the transnational parties, via increased internal party cohesion and inter-party competition.