The author presents the leading theories of the European integration, which have as ideational roadmaps steered the process of integration. The theories are divided into three groups: international law theories, statist theories and the sui generis theories in between. They have all impacted on the process of the European integration, even if not to the same extent. Over the last decade, as the European Union has been riddled by many crises, these theories have also served as a stepping stone for the development of the visions of the future of the EU. The international law theory has thus engendered the status quo ante political vision; sui generis theories breathed life into the status quo vision, while the reformed EU constitutionalism undergirds the reformist political vision of the EU. The latter, which should give rise also to the political union necessary for a democratic legitimation of the competences that the EU cur- rently has and is yet to receive has clear descriptive, explanatory and normative advantages over the alternative political visions of the EU and their underlying theories. Provided, of course, that the EU develops into a union of 27 Mem- ber States and the supranational level connected into a non-statist federation. This type of constitutional construction necessitates for its efficient and viable functioning a rigorous integral theory for the future of the EU. It is the latter’s development that presents the greatest intellectual challenge in the field of EU law and politics.

Keywords: European Union, theories of European integration, international law, state, sui generis nature of EU, federalism, constitutionalism, integral theory of the future of the EU.