In May 2004, the enforcement of the EC competition rules changed radically. Under Regulation 1/2004, the national competition authorities (NCAs) of all 27 Member States were called upon to actively participate, together with the European Commission, in the enforcement of Articles 81 and 82 EC. In order to ensure the efficient and consistent application of the law, Regulation 1/2003 provides for a number of co-operation mechanisms, at the heart of which lies the European Competition Network (ECN). The ECN acts as a common forum for information exchange, co-ordination, and discussion. This book provides an in-depth analysis of the rules governing cooperation within the ECN. The book focuses on horizontal cooperation between the NCAs of different Member States. It starts by looking at the arrangements for case allocation, discusses the rules on information exchange, and also deals with the consultation procedure prior to the adoption of final NCA decisions. The existing rules are assessed not only in the light of their purpose, ensuring efficiency and consistency in the application of the law, but are also reviewed against the requirements of the EU Charter and the European Convention on Human Rights. Particular attention is devoted to issues arising in the event of parallel procedures by several NCAs. This includes the applicability of the ne bis in idem principle and the question concerning which duties of the NCAs result from the loyalty obligation enshrined in Article 10 EC. Finally, the book explores whether the ECN concept of cooperation and networking could be applied more broadly in the context of European integration.