After the attacks of 9/11 terrorism and other forms of transnational risks of violence dominated official security policy. Researchers at the Institute for Peace Research and Security Policy at the University of Hamburg investigated the consequences of this change for security governance in a multi-annual research program. Case studies show that transnational security policies changed, but that national governments remained dominant. In other words, the transnationalisation of threat perceptions only led to a limited internationalisaton of security policies. The volume presents results of the research program. It combines conceptual work on security governance with empirical research, for instance on counterterrorism, changing perceptions of security in international organizations, such as the European Union and the Organization for Security and Co-operation in Europe.