The aim of this paper is to analyse how the European Court of Justice (ECJ) has been a fundamental factor in the integration process of the European Union, in spite of the obstacles posed by the intergovernmental dynamics that have traditionally hindered the construction of a stronger, cohesive and more integrated Europe. Important principles such as direct effect or supremacy of EU law have been developed through ECJ rulings and case law, even when such principles were not literally foreseen in the foundational Treaties. Therefore, this paper argues that the role and power of the Court as an “indirect law-maker” have been essential for the construction of the European Union, and this has been possible due to the complexities and weaknesses of the legislative process involving the three main decision-makers: the Commission, the Council of the EU, and the European Parliament.