Participating in a cartel as well as contributing to the implementation of a cartel can lead to substantial fines. The European Court of Justice recently confirmed that a consultancy firm may be held liable for infringement of the cartel prohibition where it actively contributes, in full knowledge of the relevant facts, to the implementation and continuation of a cartel among participants active in a different market. According to the ECJ, the consultancy firm played an essential role in the cartel infringement by organising meetings where it was present and participated actively; collecting and supplying sales data to the cartel participants; offering to act as a moderator in the event of arising tensions; and encouraging cartel participants to compromise, and it received remuneration for all of these activities. Companies should be aware that the cartel prohibition covers all levels of cartel participation, whether active, passive or supportive.