'The paper seeks to identify concepts and theories to analyze and explain the relationship between the member states and the European Union. It mainly adopts a top-down perspective looking at how the European Union has affected the member states and to what extent it has changed their domestic institutions, policies and political processes. What is the effect of the European Union on the member states? The paper reviews the existing literature, which offers different insights on each of the three questions. While by now most students of the European Union agree that its effect on the member states is differential, there is still little consensus on how to account for variation in the processes, degrees and the outcomes of domestic change. N...