This thesis seeks to explain why two states, faced with a similar terrorist threat, perceiving it in a similar way and drawing the same broad implications for their counterterrorist policies, have nevertheless displayed significant differences in their responses to that threat. Claiming that France and the UK have faced a similar type and level of threat from Islamist terrorism in recent years, the thesis shows that British and French officials’ similar perceptions of the militants facing them (as unrestrained and focused on producing mass casualties) has led both states to adhere to a preventive logic of counterterrorism. In this context, there has been some convergence between the two states’ counterterrorist legal frameworks and operations. Notwithstanding these common elements, I argue that different domestic institutional conditions and normative contexts in the two states are proving to be powerful factors for divergence between their respective responses to Islamist terrorism. Institutional legacies - at both the state and organisational levels - affect whether and how counterterrorist agencies and different branches of the state work together in the fight against terrorism. The contrasting institutional conditions found in Britain and France mean that the two states have achieved different types and degrees of co-operation between counterterrorist intelligence, law enforcement and prosecution. The thesis also argues that a normative consensus on security issues in French society allowed the authorities there to be considerably more draconian in their application of terrorism powers than the British, who were constrained by the high level of norm competition in their society on these issues. These different normative contexts also account for the significant differences between the two states’ operations against suspected Islamist terrorists and their supporters. By drawing on neo-realist theory, new institutionalism and constructivism in a study of a top priority area of states’ security policies, the thesis has implications for a number of theoretical debates as well as for understandings of western responses to terrorism in the 'post-9/11' world.