Chapter 8 reviewed the increasing impact of the State aid prohibition on EU corporate tax law. This Chapter showed how this prohibition has affected the design of a country’s tax system and certain aspects of its administration. The central role of the Commission and, to a lesser extent, national courts was explored. Relevant soft law and some important case law in this area were considered, before some of the high profile investigations focusing on tax rulings were analysed (e.g. Belgian Excess Profits, Staburcks, Fiat, Apple etc)