This chapter examines how the EU and the US cooperated on internal security matters, analysing the EU–US negotiations for the mutual legal assistance (MLA) and extradition agreements from a regime-theory perspective. It shows that the regime-formation process in the sub-case of judicial cooperation was based on joint gains expected to be derived as a result of the two sides' cooperation and on mutual adjustment, resembling thus the interest-based patterns of regime formation. The chapter also shows that before the 9/11 attacks the issue of transatlantic judicial cooperation against transnational threats such as organised crime, drug trafficking, and terrorism was mainly framed as a bilateral problem that the US had to address in its bilateral relations with the EU member states. The judicial regime-formation process between the EU and the US started immediately after the terrorist attacks of 9/11. One of the EU aims for the MLA and extradition agreements was the protection of human rights and fundamental freedoms.