Abstract The governance of the European Union has been changed through its responses to international terrorism. The analysis of those changes is grounded in an examination of the different perceptions of the phenomenon in academic and political debate. This introductory article traces the most relevant changes across competences, policies and governing modes and highlights dynamics applicable to other areas of EU activity: cross-pillarization, the growth of horizontal governing networks, co-operation outside the treaty framework and the impact of third countries on EU policy-making. The article puts forward a three-pronged constructivist framework to understand better the main dynamics and factors underpinning the various forms of change, in particular why the emphasis has been on co-ordination and information-sharing rather than on supranational integration. Performance issues are then critically assessed, both in terms of whether the new measures, competences, instruments and resources are likely to be effective as well as with regard to the intended and unintended harmful effects for the civil and democratic rights of EU and third-country citizens.