This book analyses the cooperation between the European Union and the United States on internal security and counter-terrorism since the 9/11 terrorist attacks. In particular, four areas of cooperation are examined: customs and supply chain security; judicial cooperation (the mutual legal assistance and extradition agreements); law enforcement cooperation (the Europol-US agreements); and the EU-US agreements for the sharing of air passengers' data (PNR agreements). These cases are analysed through a conceptual framework based on the theories of international regimes, with the data being drawn from an extensive documentary analysis of media sources collected through the 'Nexis' database, official documents, and from 13 semi-structured elite interviews with US and EU officials. The book argues that the EU and the US have established a transatlantic internal security regime based on shared principles, norms, rules, and interests. While at the beginning of this process the EU had a more reactive and passive stance at the later stages both the EU and the US were active in shaping the transatlantic political agenda and negotiations. The book demonstrates how the EU has had a much more proactive role in its relations with the US than has often been assumed in the current literature. This book will be of much interest to students of EU policy, foreign policy, international security and IR in general.