Under what conditions does the internal cohesiveness of the Euro- pean Union (EU) determine its external effectiveness? In a first step, this introduc- tion probes the frequent assumption that the more cohesive the EU presents itself to the world, the more effective it is in achieving its goals. The empirical contributions to this collection, which range from trade to foreign policy, highlight instead three configurations of internal cohesiveness and external effectiveness: internal cohesive- ness has a positive impact on external effectiveness; internal cohesiveness has no impact on external effectiveness; and internal cohesiveness has a negative impact on external effectiveness. The international context in which the EU operates, including the bargaining configuration and the policy arena, functions as an inter- vening variable in these causal links. In a second step, this introduction launches a research agenda aimed at explaining these patterns more systematically and deter- mining the impact of cohesiveness on effectiveness.