In the 21st Century, P/CVE has become one of the most popular counter-terrorism policy areas. P/CVE, Preventing and/or Countering Violent Extremism, is a pre-emptive approach to impede mobilization against political violence before it happens. It should be obvious that P/CVE constitutes an important departure point from national 20th century approaches to internal security. Firstly, it moves away from traditional counter-terrorism focused on police persecution, punitive or repressive measures, and judicial intervention. But secondly and most importantly, because P/CVE has allowed the European Union a front-row seat to enact transnational influence on a previously only national domain: internal security. Indeed, as part of counter-terrorism strategies globally, P/CVE is supported by national actors even when it implies allowing international institutions, like the European Union, some influence in managing their interior security. This thesis is an analytical account of the development and current governance of P/CVE in the European Union, tracking the transnational institutional involvement and influence of the EU in shaping the policy design of and final national implementation in this area of counterterrorism. It puts forward the following theoretical contributions. Firstly, that the EU re-framed our understanding of terrorism in order to introduce P/CVE as an adequate counter-terrorism solution. That this re-framing of terrorism required of an institutionalization process, most active between 2006 and 2011 and culminating in the creation of the RAN agency. That the success of the institutionalisation and Europeanisation process of P/CVE was possible only because of a collective EU agency. And finally, that the introduction of P/CVE as a counterterrorism policy sub-field indirectly granted the EU indirect governance capabilities on internal security matters. Content wise, the thesis starts with a methodological reflection on remote (telecommunication) methods for qualitative elite interviews. Then, this doctoral research examines, firstly, the institutionalization of P/CVE at the European level. And secondly, it interrogates the capacity of the EU to transnationally orchestrate a certain level of governance of national P/CVE regimes. Finally, this thesis maps an emerging transnational threat, online radicalization and right-wing extremism, long ignored by EU policy and governance, and provides policyactionable research on the topic.