The involvement of regions in the European Union is a key element in the integration of federal States and in the process of European construction. The participation of the regions is a key element of the multilevel constitutionalism. It should be recalled that the European integration process involves a transfer of powers to Europe that often affects the powers of the regions. This course examines the representation of regions in the European Union, their participation in the upward phase (formation of Union Law), and the downturn or application of European Union Law. As an example, we particularly study the Spanish and German cases.