To date, the European Court of Justice (ECJ) has decided around 400 cases in the field of direct taxation and has thus considerably shaped the applicable tax law in the EU Member States: Apart from necessary changes in the national law of the Member State that was immediately affected by the legal dispute, many rulings had a meaningful impact on other Member States’ national tax laws as well. In order to create a broader understanding of various dimensions of the ECJ case law, this project aims to give an overview of Member States’ reactions to selected ‘landmark’ decisions of the ECJ in the area of direct taxation. Furthermore, the effects of these reactions will be quantified using the Devereux/Griffith model to show to which extent the ECJ case law has led to a reduction of tax distortions in the EU internal market.