The article accounts for the emergence of an external dimension of EU internal policies and focuses on the migration, asylum and border management domains. Building on Bretherton & Vogler (2006), it assesses and explains the forms and extent of EU’s external engagement. Firstly, it assesses the degree of EU actorness – that is, the extent to which the Union has become an actor in global politics - in the researched fields in terms of opportunity, presence and capabilities. Secondly, it attempts to explain the emergence of such EU external engagement. While the article defends that the treaty objective of creating an European Area of Freedom, Security and Justice (article 3 TEU) makes EU external action in the migration, asylum and border control domains necessary, it also acknowledges the role of external triggers, such as the so-called 2015 ‘migration and refugee crisis’. On the one hand, external events can provide scope conditions accelerating the pace by which EU external action is pursued. On the other hand, they can also trigger the comeback of hard security concerns linked to claims for ‘more national sovereignty’, thus constraining the development of EU (external) action. The analysis carried out shows that the degree of EU actorness varies considerably across the three domains researched. On one extreme of the continuum, neo-realist understandings of increased arrivals of foreign citizens as hard security challenges to member states account for the enhanced interest in cooperation at the intergovernmental level. Such interpretations explain why the most established domain for EU external engagement is the cooperation with third countries on border management, as it is shown by the expansion of the mandate of EU agencies such as Europol and Frontex, or by negotiation by the European Commission of international agreements containing clauses on readmission of third country citizens irregularly residing in the EU to their countries of origin (the Mobility Partnerships). Less developed, yet quickly evolving, is the EU external dimension of migration policies. Functional concerns related to the need to acquire highly specialized (and relatively cheap) workforce towards an efficient functioning of the single market have prompted a remarkable the law-making in this area, as shown by the EC directives and regulations aimed at third country students, unpaid trainees, voluntary workers and big corporations’ staff. While the neo-functionalist logic explains the emergence of the external dimension of EU migration policies, it may also explain this development is currently limited to certain areas within the policy area. A comprehensive approach to migration policy has not proved possible to achieve so far, due to institutional and bureaucratic limitations, as well as the different agenda of national policymakers. On the other extreme of the continuum, liberal theories and constructivism account for the constraints currently limiting the full development of EU actorness in asylum policies. Several member states’ civil societies do not perceive prospects for absolute gains deriving from cooperation at the EU level. Constraints to the establishment of international institutions regulating asylum policies are thus understood through the lack of strong societal pressures at the member states level. Therefore, while the article puts forward a combination of neo-functionalist, neo-realist, liberal intergovernamentalist and constructivist explanations to account for the emergence of an external dimension of migration asylum and border management policies, as wells as for the obstacles opposing that process, it also provides evidence that external triggers such as increased migratory pressure lead to internal policy dynamics with positive or negative impacts on the shift of actorness from EU member states to the EU itself.DOI: http://dx.doi.org/10.5755/j01.eis.0.11.18956