In this chapter, Rita Szudoczky examines the relationship between primary and secondary EU law. While primary and secondary Union law can interact with each other in numerous ways, the fact that there is a hierarchical relationship between them stems from the basic constitutional order of the Union. Despite this constitutional tenet, the case law of the Court of Justice is inconsistent as regards the relationship of primary and secondary law. What clearly stands out from this case law is the reluctance of the Court of Justice to exercise an effective substantive review of the legality of secondary legislation in light of primary law and thereby enforce the hierarchy between the two. This chapter examines different lines of the case law – in general, and with a special focus on the tax case law – in order to demonstrate how the Court of Justice avoids directly addressing a conflict between primary and secondary law. It is argued that the Court of Justice needs to strengthen the constitutional review of secondary law while, at the same time, maintain the institutional balance within the Union. It is especially important to do so in the area of taxation, where, on the one hand, the degree of harmonization gradually increases and, on the other, harmonization measures expand beyond their traditional sphere.