Abstract: In recent years, the European Court of Justice (ECJ) has invalidated many income tax law provisions of European Union (EU) member states as violating European constitutional treaty guarantees of freedom of movement for goods, services, persons, and capital. These decisions have not, however, been matched by significant EU income tax legislation, because no EU political institution has the power to enact such legislation without unanimous consent from the member states. In this Article, we describe how the developing ECJ jurisprudence threatens the ability of member states to use tax incentives to stimulate their domestic economies and to resolve problems of international double taxation. We conclude that the ECJ approach is ultimately incoherent because it is a quest for an unattainable goal in the absence of harmonized income tax bases and rates: to eliminate discrimination based on both origin and destination of economic activity. We also compare the ECJ's jurisprudence with the resolution of related issues in international taxation and the U.S. taxation of interstate commerce, and we consider the potential responses of both the European Union and the United States to these developments.