The aim of this article is to determine the EU's effect on British–Irish and Greek–Turkish cooperation. It examines the EU's actual and potential role in fostering British–Irish and Greek–Turkish cooperation as well as its policies to Northern Ireland and Cyprus themselves. Domestic bureaucratic capacity and institutional design are put forward as explanations for the success of the British–Irish relationship rather than EU membership per se. In the case of Cyprus, the article suggests that increased bilateral cooperation with respect to disputed territory is unlikely to occur in the absence of faster bureaucratic change in both Greece and Turkey. EU membership is not a causal factor in the evolution of cooperation and conflict resolution.