The decisions of the European Court of Justice (ECJ) have impacted the design of the EU member states’ national (individual and corporate) tax systems in several areas (e.g. exit taxation, thin capitalization rules, cross-border loss compensation). The ECJ case-law has an increasing influence. It already decided more than 200 cases related to direct taxation. Therefore this research project aims at fostering a far-reaching and comprehensive understanding of various aspects of the case-law. It is of interdisciplinary nature and involves political scientists, tax lawyers and economists. One of the research questions is whether judgments are influenced by political determinants, e.g. if the economic size of a member state has an influence on the court’s decision. The legal analysis of judgments will mainly focus on the applicability of different justifications and moreover on their consistency and development up until today. The project further examines how member states react to a judgment by amending their legislation. This is important because this information serves as a basis for an analysis about whether the ECJ case-law encourages or discourages further European integration.