Recent years have seen the rise of EU State aid law as a crucial component of the European economic constitution. To date, however, the literature has neglected the contribution of this area of EU law to the internal market. This book seeks to fill this gap in our understanding of the economic constitution by exploring the significance of State aid law in addressing questions that go to the core of the internal market project. It does so by examining the case law relating to three different activities that Member States engage in: market participation, market regulation, and funding for Services of General Economic Interest. Each of these areas offers insights into fundamental questions surrounding the economic constitution, such as the separation between the State and the market, the scope for Member States to engage in regulatory competition, and the tension between market and nonmarket concerns.