Is the European Union a legislative giant on clay feet? Is it true that the Union has, with some specific exceptions, no original competence to implement European law? The presentation analyses the structure of the Union’s “executive federalism” in three steps. After a comparative constitutional section on the centralized (American) and decentralized (German) enforcement systems of federal norms, the constitutional foundations of executive power in the European Union are explored. Will Article 291 TFEU provide a reformed textual base for the (new) Union’s executive powers? A third section then examines existing constitutional limits to the national (decentralized) and European (centralized) enforcement of European law. Beginning with the decentralized implementation mechanism, a first part of this section looks at the substantive, procedural and morphological limits on the national implementation of Union law. A second part of that section changes perspective as it investigates the constitutional limits on the executive powers of the Union in the form of, for example, the principle of subsidiarity. An excursus briefly analyses the phenomenon of “mixed administration” through a federal lens. And a conclusion finally argues that the Lisbon Treaty will remedy, to some extent, the lack of clear constitutional foundation of Union executive power.