This conclusion presents some closing thoughts on key concepts discussed in preceding chapters of this book. The book examines how the EU and the US negotiated a number of agreements in the period after the 9/11 attacks in the area of internal security. It explains transatlantic regime formation in the issue-area of internal security was ultimately a negotiated process rather than an imposed process by the US. The book presents a comparison of the cases examined, and the implications for EU–US internal security and counter-terrorism cooperation and for EU counter-terrorism. It explores the cooperation between the EU and the US on internal security matters could be conceptualised as a nascent regime and has amounted to a de facto integration project between the two sides. The concern of both the EU and the US for the implications of security measures for the private sector and the economy was reflected in the principle of trade facilitation that the two sides have adopted.