This paper explores the evolution of operational responses to Islamist terrorism in Europe since 1995, with a particular focus on Britain and France. In both cases, the security policies that guide counterterrorist operations are decided by a combination of government officials and police and intelligence officers. The paper seeks to explain the mix of similarities and differences between the British and French cases and draw the implications for European counterterrorism. A simple concept of threat perception, drawn from neo-realist theory, can explain key aspects of Britain and France’s internal security operations against Islamist terrorism. Both states mounted extensive and forceful counterterrorist operations when they perceived a high level of threat to their own territories. Nevertheless, a realist perspective cannot explain other significant characteristics of the cases, such as why France resorted to indiscriminate arrest operations while Britain refrained from such actions. The UK has also been less invasive than France in its operations against alleged supporters of Islamist terrorism. Drawing on constructivist theory, the paper argues that these differences flow from contrasting norms, based on historical experiences, found in the two states. Based in part on interviews with 41 counterterrorist officials and other informants, the paper forms part of a larger book project, which seeks to move the counterterrorism literature beyond its traditionally descriptive orientation towards a greater focus on the development of empirically-grounded explanatory theories.