Nowadays in competition law, the detection of cartels and collusive behavior is as important as the effectiveness of the legal and financial consequences on such behavior. The main questions in this study are, how competition agencies can prepare themselves for a system that allows sharing of information related to a leniency proceeding to private parties (for private enforcement), without losing any attractiveness of their leniency programme and how a European Leniency System can be build up in a most effective way. Moreover, a future European "One- Stop-System" is discussed and an outlook given.