In a series of judgments regarding direct taxation and indirect taxation as well as other areas of EU law the CJEU has established the notion that there exists a “general principle” prohibiting individuals from relying on primary or secondary EU law for improper purposes. This article tries to disentangle this general concept, outlining the different methodological, institutional and substantive functions the concept of “abuse” fulfils in EU tax law. It makes a difference whether the fight against abusive transactions comes up as a means to reject a taxpayer's entitlements on the basis of a directive, as a ground of justification when infringements on fundamental freedoms by domestic legislation are at stake, or when the European legislator embarks on policies addressing “aggressive tax planning”. It is shown that each of these contexts requires a different understanding of “abuse”, in particular when the “economic substance” or the “commercial motives” of a transaction are under examination.