This book analyses multi-level governance in competition policy, or "antitrust federalism" as it is called by students of competition policy, in the US and the EU from a comparative perspective. The book compares how competition laws and authorities of different levels - the federal and the state levels in the US and the national and the supranational levels in the EU - interact with each other. The EU and the US stand among the strongest existing examples of multi-level polities and they developed mature competition policies. Despite such similarities, however, recent developments imply that they are moving in different directions in the field of antitrust federalism. Inspired by these divergent policy developments taking place at both sides of the Atlantic, the book addresses three principal research questions: firstly, what are the key similarities and differences between the US and the EU in terms of antitrust federalism; secondly, what are the reasons for differences (if any), and finally, can the US and the EU draw any policy lessons from each others' experiences in antitrust federalism? The book is essentially multidisciplinary in nature and it aims to initiate a dialogue between the law and political science literatures in its field. The book argues that the legal literature of antitrust federalism has employed out of date regulatory competition models which do not reflect the complexities of policy enforcement in modern multi-level polities. The book suggests that policy network models provide a more suitable framework for this analysis; and it critically reviews the British and Continental European policy network models. The book uses the common conceptual framework of European policy network models as the main analytical framework in the analysis of antitrust federalism. However, the book also shows that constitutional courts significantly affect different network designs in different polities through interpretation of constitutional power sharing and exercise mechanisms; and it critiques the political science literature for overlooking such essential role of the constitutional courts in building network models.