This paper examines the extent of the use and content of unwritten general principles of EU law in EU antitrust law. In particular, three unwritten general principles will be explored in more detail: effectiveness, proportionality and non-discrimination. The contribution will start by examining the explicit use of the general principle of effectiveness in antitrust law proceedings and rules related to its enforcement. The second part will explore the role of proportionality, both within the EU antitrust procedural law and the substantive assessment of an antitrust case. Finally, it will focus on the role of non-discrimination within the substantive assessment of EU antitrust cases. The paper shows that these principles are affirmed, expressly or implicitly, not only in procedural but equally in substantive antitrust law. Yet, their role and actual use varies significantly between antitrust procedure and the substantive assessment of a case. While these principles have a more explicit role in antitrust procedure, their role is less pronounced in the substantive assessment of antitrust cases. The assessment of these principles in the substantive case might also be different as the principles can be applied in a way that is open to economic reasoning and the EU Courts afford a high margin of discretion to the Commission in making complex economic assessments.