The issue of international antitrust enforcement is high on the agenda for both developed and developing countries. Bilateral cooperation between antitrust agencies, in particular the European Commission and US agencies, is the focus of this new work. It first shows how bilateral cooperation was developed as a response to the limits of the unilateral and extraterritorial application of national competition laws, and how it has evolved from an instrument initially designed to avoid conflicts into a tool aimed at coordinating joint investigations of international competition cases. It then considers how bilateral cooperation could be used optimally, by analysing two forms of advanced cooperation: the exchange of confidential information, and positive comity, which is the only satisfactory answer competition law can provide to market access cases. It shows that the use of such instruments is limited by significant legal and political obstacles, even in the context of the exemplary EC US relationship. The book therefore argues that the efficient use of bilateral cooperation will be limited to a small number of well-established competition agencies. If international anticompetitive practices are to be efficiently addressed by an increasingly large and heterogeneous group of competition agencies, horizontal cooperation between antitrust agencies must be complemented by a multilateral and supranational solution going beyond proposals currently put forward. The book concludes that only the WTO and its dispute settlement system could provide the basis for such a system.