Prior to 11 September 2001, the counter-terrorist responsibilities overseen by the European Union (EU) were relatively unimportant. Since then, however, member states have decided to engage the EU in a larger number of counter-terrorist issues and, in some cases, empower it to undertake substantial tasks. The EU has thus become an important player in counterterrorism in Europe; notwithstanding the fact that the major actor remains the member states themselves. However, this increase in EU engagement on counter-terrorist issues has varied enormously from one policy area to another. This asymmetric increase lies at the centre of my research question: since 11 September 2001, why have member states conferred important anti-terrorist responsibilities to the EU in some areas - for instance, judicial cooperation in criminal matters - and less significant in others, such as policing? I address this question by investigating the agenda-setting and decision-making processes of two specific EU decisions in each of my two policy area cases (2001-2007). In each case, one decision constitutes a large increase of EU engagement and the other represents a small or zero increase. The two cases are: Judicial Cooperation (European Arrest Warrant and the European Evidence Warrant) and Police Cooperation (EU ‘Prüm Measure’ and failure of the Commission’s proposal on the Principle of Availability). In order to explain the research puzzle, I apply a modified version of John Kingdon’s ‘Three Strands Model.’ This enquiry sheds light on the relative influence on decision-making of the occurrence or non-occurrence of a major terrorist attack (i.e. changes in the addressed problem) and the entrepreneurship of the European Commission or of the member state holding the rotating Presidency of the EU Council. The EU member states are the central actors and their preferences are analysed as a means to understand the role played by the logic of consequentialism and the logic of appropriateness, respectively.