European integration has had an ever deepening impact on the member states. The first wave of research concerned the process of institution building and policy developments at the European Union (EU) level. The second wave, on Europeanization used the resulting integration as an explanatory factor in understanding domestic political change and continuity. What is now necessary is to link our understanding of these ‘bottom-up’ and ‘top-down’ processes of integration and Europeanization. This book argues that a third wave of research on the EU is needed to adequately understand the increased interconnectedness between the European and national political levels. We posit that this third wave should be sensitive to the temporal dimension of European integration and Europeanization. In particular, we ask: how has Europeanization affected current modes of integration and cooperation in the EU? One of the key findings of the book is the astonishing variation in modes of cooperation in the EU. We suggest that this variation can be explained by taking into account the sources of legitimacy on which cooperation and integration are based. We argue that whereas economic integration could be sufficiently backed by output legitimacy, deeper integration in other areas requires a degree of input legitimacy that is currently lacking in the EU. Therefore, non-economic integration is often taking forms of looser types of cooperation, such as the open method of coordination and benchmarking. We elaborate on this speculation in the conclusion and believe that it should be part of the future research agenda.