Early theorists of European integration expected that political actors such as interest groups would shift their political activities towards the new centre, leading to further political integration in Europe. Half a century later, we assess this expectation and find large variation in the extent to which national interest groups focus on European Union (EU) legislation and have shifted their political activities to Brussels and Strasbourg. What explains this variation? We propose a series of hypotheses that focus on group type,
group resources, policy held and the size of the group's home country. Using data on 880 national associations, gained from a survey of interest groups in five European countries, we find support for these hypotheses. The paper has implications for the literatures on lobbying, decision-making in the EU, Europeanization and theories of European integration.