The article aims to explore the utility of Europeanisation as a concept to grasp the interactions between national and European levels. The article illustrates how the EU impacts on the national level of policy, polity and the politics of Member States and assesses how the role of nation states within the European political system has changed as a result. First, the existing definitions of Europeanisation are critically assessed, contextualised and delimited. Initially developed for communitised policy areas in the first pillar, Europeanisation is defined as an interactive, ongoing and mutually constitutive process of ‘Europeanising’ and ‘Europeanised’ countries, linking national and European levels. Defining Europeanisation as ‘domestic change’, the article then discusses mechanisms, objects and forms, as well as the criteria and conditions of change. The article subsequently seeks to clarify the validity of the Europeanisation concept in capturing the increasing interwovenness of national and European spheres in intergovernmental policy fields situated in the second pillar of the EU, that is, foreign and security policy. Inherent methodological challenges, mainly due to the deficient delimitation of Europeanisation and the intergovernmental character of this policy field are discussed as well as the particularities of its applicability in this unique policy area.