The article considers two current challenges to the European Commission’s cartel enforcement - first, EU leniency trends and second, multiple recent court annulments of Commission fines. In particular, although leniency policies play a fundamental role in detecting and deterring cartel activity, the effectiveness of the EU regime has increasingly been called into question. Separately, the Commission has seen a number of its fi nes relating to cartel violations annulled due to a lack of suffi cient reasons. Together, these trends suggest the Commission may receive less co-operation from alleged cartelists and a greater willingness to challenge infringement decisions, particularly the level of fi nes imposed.