The European Commission decides collegially, meaning that every member of the college is equal in the adoption of decisions. The principle of collegiality is a core legal principle of the Commission’s internal decision-making and dictates how the institution legitimately develops, implements and enforces policy in a multinational polity. Because of collegiality, Commissioners bear collective responsibility for the decisions they adopt. This in turn guarantees the Commissioners’ independence from Member States and acts as a safeguard to the Commission’s supranational character.  My research focuses on the legal substance of the principle of collegiality, its implementation in the Commission’s decision-making and how collegiality has evolved to adapt to changing institutional and political dynamics. The legal analysis of the principle is complemented by empirical research to retrace implementation in the different layers of decision-making (college-services) and according to the different functions performed by the Commission in its enforcement, administrative and legislative activities. Targeted case studies in specific functional areas help assess how the legal definition of collegiality matches up with reality.The thesis argues that although collegial procedures permeate every aspect of the Commission’s decision-making, there is a gap between the legal concept of collegiality and its actual practice. Formally, the college of Commissioners adopts all decisions, regardless of whether they are legislative proposals or competition decisions. Behind the scenes, however, the principle is implemented with different intensities and in different ways depending on the functions performed by the Commission. These variations raise the question of how to ensure the unity of the Commission as a collegial body despite the diversification of decision-making functions. They also encourage an examination of how the principle of collegiality affects the Commission’s role in the current EU institutional, legal and political setting.