How has the evolution and transformation of the Common Market affected the legal concept of State aid? How has State aid adapted to the development of the European Union? These questions and more are answered in Juan Jorge Piernas López's examination of the historical, political, constitutional, and economical events that have affected the development of State aid in the EU. Examining three key, interwoven arguments, this book provides a richer understanding of current formulas which depict the concept of aid through the prism of policy and enforcement considerations. First, the book demonstrates that the concept of aid is a 'living instrument' that has been applied in accordance with the main policy priorities of the European Commission. Second, contrary to what has been affirmed in other literature, the evolution of this concept has been influenced by the broader advancement of the case law of the Court of Justice in different periods of the integration process. Third, the author contends that the study of the evolution of the concept of aid in light of policy and case law provides a holistic outlook valuable to the decision making process of difficult cases. In this regard, the book provides criteria to interpret and discuss cases including Sloman Neptun, Philip Morris, and Azores, beyond the analysis traditionally adopted in this field.