With its judgment in Engie, the CJEU has annulled yet another Commission decision on the state aid compatibility of Member States’ tax rulings. The Grand Chamber found that the Commission and the General Court had incorrectly identified the reference system of taxation made up of the ordinarily applicable national law of Luxembourg. When it comes to the interpretation of national law in the state aid procedure, the interpretation provided by the Member State is generally decisive. A high evidentiary threshold applies if the Commission wants to depart from this interpretation. This holds all the more true for Member States’ General Anti-Abuse Rules (GAARs), which are by their very nature general. The judgment could have wide-reaching implications for the enforcement of state aid law by the Commission regarding all types of cases where the interpretation of national law is not altogether clear. For example, the interpretation of national law is a key point in the UK Controlled Foreign Company (CFC) case, which is why AG Medina in her conclusions draws heavily on the Engie judgment. As a result, the Commission will probably lose the UK CFC case.