The present contribution addresses the excessive amount of discretion left to the EU Commission (and Courts) in defining the enforcement priorities in the field of EU State aid Law, by singling out one element of the (inherently vague) the notion of State aid, namely the effect on trade between member States. The approach taken by the Commission’s practice and the ECJ case law in this field ends up building a rather unpredictable legal framework. This risks unreasonably undermining both member States’ legislative choices in fields not necessarily falling under an EU competence, and undertakings’ legitimate expectations.