This paper investigates the legal nature of the principle of collegiality as a principle of EU law. It mainly asks the following questions: what kind of legal principle is collegiality? Can it be considered as a general principle of EU law in its own right? And how does it integrate the EU legal system? Collegiality is the main principle according to which the college of Commissioners adopts decisions and as such it identifies a key feature of the Commission’s internal decision-making. Yet its legal status has rarely been addressed by legal scholars and remains largely unexplored. The paper argues that collegiality constitutes a general principle of EU law in its full autonomy. However, because of its specific nature, its legal status is fundamentally affected by an underlying ambiguity, which resides in the dual functions performed by collegiality as a procedural principle of administrative law and as an institutional principle for the Commission’s organisation. The Court has recognized concrete legal effects to the principle of collegiality as a procedural principle protecting the interests of individuals affected by the Commission’s action. Beyond that, however, collegiality is also a broader principle determining the institutional articulation of the Commission’s work both at the college and at the administrative level. It thus constitutes a key element of EU Governance.