This chapter introduces the theoretical framework that is used in this book, which is based on the theories of international regimes. One of the main issues of regime theory is regime formation, or in other words how international institutions, agreements, and governance systems are negotiated and concluded (Young 2005: 92–95). Regime theory has been previously used in analysing international agreements on environmental problems (Dimitrov 2003; Andresen et al. 2013), economic and trade issues, technological issues and telecommunications (Ruggie 1975; Cogburn 2003; Raustiala and Victor 2004; Park 2009), security issues and counter-terrorism, and human rights. In transatlantic relations, in particular, Grieco (1990) has previously looked at US–EC trade relations from a realist standpoint, while Rees (2006) has focused more on the topic of regime maintenance and persistence by looking at how the EU and the US used and adapted previously established avenues of cooperation for counter-terrorism purposes after the 9/11 attacks. Therefore, theories of international regimes can provide a fruitful conceptual background for analysing the cooperation and negotiations between the EU and the US, and thus for answering the main research question of this book of how and why the two sides negotiated agreements in the area of internal security.