This article explores the recent and significant growth of European Union cooperation on counter-terrorism by drawing upon the concept of ‘collective securitisation’. It highlights how 9/11 was a precipitating event, one which led some EU leaders to call upon the member states to develop an EU counter-terrorism policy and to step up counter-terrorism cooperation with the United States. Various counter-terrorism legislative instruments have since been adopted, defining terrorist acts and having a significant impact on national counter-terrorism policies. 9/11 was therefore used by some actors to convince the EU member states that they all faced one collective terrorist threat, embodied at the time by al-Qaeda, rather than each of them facing a distinctive threat as had hitherto been a common view across Europe. This was a crucial moment, in other words, paving the way for the development of a common EU counter-terrorism policy. The subsequent institutionalisation of this cooperation, in particular through the establishment of the European Arrest Warrant, the Counter Terrorism Coordinator and the European Counter Terrorism Centre within Europol, has contributed to a routinisation of counter-terrorism practices in the EU.