Third-party complainants play a crucial role in the enforcement of competition rules.1 As the European Commission itself noted, ‘[t]he complainant can help the Commission in supplying evidence of the anticompetitive practice and thus in establishing the infringement’.2 Given the authority’s limited resources, which result in a limited ability to conduct an ongoing screening of several markets, complaints are the most important source of information for the Commission. Unsurprisingly, the landmark decisions issued in recent years, e.g. Google Shopping, were driven by complaints filed by several market participants.3 ... Against this background, one should expect the legal position of complainants during antitrust proceedings before the Commission to be effectively protected. A lack of such protection can result in reduced incentive to file a complaint. This is even more relevant for less affluent victims for whom it is sometimes difficult to obtain specialised legal support or to make successful private enforcement claims and who are therefore even more dependent on the enforcement by the Commission. On the other hand, there are arguments for limiting the procedural rights of complainants, which are focused on the efficiency of proceedings and good management of resources.4 In theory, a balance is struck between incentivising complaints and the usage of the Commission’s resources through prioritising cases or choosing criteria to reject specific complaints.5 We would like to take a critical view of this balance through the lens of effective judicial protection of complainants’ rights.6