Article 6 of the EU Anti-Tax Avoidance Directive (ATAD) introduced an EU law obligation for Member States to include a general anti-abuse rule (GAAR) in their corporate tax systems (ATAD GAAR). This article summarizes the findings of a PhD research of the ATAD GAAR. The main conclusion is that the ATAD GAAR codifies the existing general anti-abuse concept of EU law whilst it at the same time expands the reach of this concept to also cover non-harmonized areas of corporate tax. This means that Member States must prevent the abuse of their entire corporate tax systems as if it concerned abuse of EU law. The significant consequences of these changes are analysed, including the timing aspects, taxes in scope, and whether the provision is a minimum standard or full harmonization. This article also analyses the procedural tax aspects of the ATAD GAAR and its impact to the state aid framework. The ATAD GAAR is subsequently criticized with reference to ideas of Karl Popper’s The Open Society. Finally, the article provides an outlook to the future of the provision.