In October 2015, the European Commission concluded that tax rulings agreed between tax authorities and Starbucks (in the Netherlands) and Fiat Finance and Trade (in Luxembourg) constituted illegal State aid, and ordered each company to repay EUR 20 to 30 million. These decisions mark a significant milestone in the use of State aid powers by the European Commission. As tax rulings are increasingly being scrutinized, this article explores the focus of the investigations, as well as how multinationals with EU-based subsidiaries can assess whether their tax rulings are compatible with State aid rules.