AbstractOpt-in or opt-out? That is the basic question to be answered. The decision to promote actions of “opt-in” type as opposed to those of the “opt-out” type, for the sake of private autonomy, does not ensure the effective application of european competition law. On the contrary, it may decrease the application’s intensity and effectiveness. Recent reforms among European state members such as in the United Kingdom, Belgium, the Netherlands, Denmark and Norway are powerful indications that the opt-out principle may result in the effective implementation of competition law. There is no doubt that a mixed system (hybrid system), providing the court with the power to decide in favour of either the opt-in or the opt-out system, will result in better implementation of competition law. At the present time there are two pending cases in England (Dorothy Gibson and Mastercard) for which the decision on opt-out or opt-in are highly anticipated. Should the court decide, in one or both of the cases, on an opt-out approach, this will bring a momentous reevaluation of the entire collective redress concept.