The introduction of a competence catalogue in the TFEU by the Treaty of Lisbon eliminated the overlap between the preclusion of national lawmaking powers attributable to the exclusive character of EU competences and that stemming from the enactment of EU legislation, thus opening the doors to scholarly investigation of Union Preemption as a general theory of the effects of EU legislation in non-exclusive competence areas. The first aim of this work is to describe what Union Preemption is(“Sein”) in the context of the EU single market, thus allowing, given an item of EU legislation, to determine its preemptive scope by reference to certain “markers”, such as its legal form and the harmonization model it embodies. Against the background of the existing application uncertainties, this work further seeks to suggest what Union Preemption ought to be (“Sollen”) to meet the current needs of European integration, taking into account the diversity of situations where preemption issues arise and the changes in the legal, political, and economic context that have occurred in the single market since 1957.