The Treaty of Rome talked of "ever closer union", and it was always assumed that all member states would move forward together in each stage of European integration. However, following "opt-outs" in the Maastricht Treaty and further enlargement, it has seemed increasingly likely that the European Union will become less uniform in future. This text explores discussion about the forms that it might take. The author shows that there has been a great deal of confusion in the usage of concepts such as a "multi-speed Europe" and "a Europe of variable geometry", which the Treaty of Amsterdam has done little to allay. After elucidating the various models of integration implied by the different terms, Junge examines the problems that will be encountered with enlargement and asks whether the "flexibility clause" will be sufficient to deal with the complexity of the European Union in the 21st Century.