Regional integration and regionalism have taken on remarkable importance since the end of the Second World War. The formation of regional blocs, regional organisations, and the institutionalisation of regional cooperation are global historical phenomena. The European Union (EU), formally established with the Maastricht Treaty of 1992, currently comprises the only example of a common market between its 27 member countries with aspects of monetary union. For 80 years, European integration has constantly changed following trends of widening (geographical expansion with the accession of new members) and deepening (intensification of integration). This relatively fast pace of empirical changes gave rise to different theoretical and conceptual explanations that account for unprecedented levels of integration and transfer of authority in Europe. In other words, when we see the creation of regional blocs, International Relations (IR) researchers began to take a closer look at the European context, giving rise to different theories of regional integration, many of which would have the ambition of universal relevance and external validity.