This thesis is concerned with the legal concept of State aid under Article 107(1) TFEU. It was born out of the concern that EU institutions and practitioners alike refer to it as if this concept had been immune to the evolution experienced by the context in which it has been applied: the Internal Market. Against this seemingly static background, the thesis argues that the concept of aid is a 'living instrument' that has been applied in accordance with the main policy priorities of the European Commission, a fact that had been underexplored in the literature and the implications of this connection seldom studied. The thesis also contends, contrary to what has been affirmed before by other authors, that 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. Thirdly, the thesis submits that the study of the origins of subsidy control in Europe, of the legislative history of today's Article 107(1) TFEU, and of the policy and enforcement considerations that have affected the development of the concept of aid is not only enriching from an intellectual point of view but also useful to decide difficult cases. In this regard, the thesis provides criteria to interpret and discuss cases, e.g. Sloman Neptun, Philip Morris or Azores, beyond the analysis traditionally carried out in this field, too often limited to the compatibility of these cases with the effects-based formula traditionally followed to define aid. Indeed, the emergence of that formula, of the Market Economy Investor Principle test, the Adria-Wien test for selectivity, the ups-and-downs of the De Minimis rule in this field, or the exclusion of regulatory measures from the notion of State resources, cannot be understood only by looking at the economic or even the strictly legal reasoning behind the relevant judgments. Instead, historical, policy and enforcement considerations are useful interpretative tools to fully grasp these developments, a point that this thesis contributes to making.