EU counter-terrorism is a hybrid policy that, despite its incremental, exogenous-shock-driven integration, has evolved from de facto intergovernmental governance to gradually heading towards harmonisation. As terrorism is not an EU competence, policy-making in the area was for many years short-term and reactive instead of driven by strategically coordinated prevention and response. Despite the bodies and mechanisms in place to facilitate information-sharing and operational cooperation, until 2015 the latter were lacking to such an extent as to cause gaps in governance and operational inefficiency, including decreased capacity for prevention and response. This study focuses on the necessity, unparalleled in any other EU policy area, for operational cross-border cooperation and information exchange. This text explores the aftermath of two critical junctures in EU counter-terrorism brought about by two sets of terrorist attacks – in Madrid (2004) and London (2005), as well as in Paris (November 2015) and Brussels (2016). This analysis is carried out through the lens of a dual theoretical framework – historical institutionalism (HI) and constructivist institutionalism (CI) – which is novel to the field of EU counter-terrorism, wherein HI has previously been applied to the policy’s scholarly analysis, however a combination of the two theoretical branches has not been utilised in this area thus far. This thesis analyses the consequences of these junctures for the development of EU counter-terrorism by enquiring into the conditions and circumstances for institutionalisation of the policy and examining its added value to national efforts. The study demonstrates the increased efficiency of cross-border and inter-agency coordination in EU counter-terrorism since 2015 due to improved institutional design and legislative frameworks, leading to further policy integration.