By examining the issue of collusion in EU and US competition law, this book suggests possible strategies for improving the antitrust enforcement against parallelism, by exploiting the most advanced achievements of economic analysis. The book contains a suggested approach to collusion, in ex ante and ex post perspectives. By moving from the analysis of the state of art, in terms of law, case law, and scholarship, Marilena Filippelli analyses inconsistencies and failures in the current antitrust enforcement toward collusion and develops a workable parameter for the issue of collective dominance. The most innovative part of this work goes beyond the analysis itself of collective dominance and involves the interference of arts. 101 and 102. The conclusion is a re-definition of the relationship between those rules - from dichotomy to redundancy. Finally, the book highlights the antitrust significance of semi-collusion, as a strategy made of collusion and competition. The author considers economic models equaling, as for the effects, collusion and semi-collusion and the case law supporting the qualification of semi-collusion as a species of collusion.The analysis involves both US and EU systems, under the highly topical economic-oriented approach. It also contains an original view of European antitrust prohibitions. Because of its contents and its approach, this book will be attractive to every academic interested in antitrust law. Moreover, the well-documented research on parallelism, involving law, case law and scholarship, makes this book interesting also for competition authorities and antitrust lawyers.