This article presents the main legal approaches used in constructing the relationships between the TEU, TEC, TEAC and TECSC and the institutions set up by them. It argues that the dominant approaches which separate the European Union from the European Communities run into serious difficulties when explaining the normative framework and the actual practice of the EU institutions. In contrast, the proposed ‘unity thesis’ asserts that de lege lata the European Union can be considered one entity from the point of view of the organisation, its actions and its law, The article develops in detail the legal premises and some of the consequences of this thesis.