Large variation exists in the extent to which national interest groups focus on European Union (EU) legislation and carry out their political activities in Brussels and Strasbourg. What explains this variation? We propose a series of hypotheses that suggest that business groups, and groups active in policy areas with high EU competence, are more Europeanized than other groups. The effect of group type, moreover, is conditional on the material resources a group possesses: we expect the difference between business and non-business groups to be largest for actors that are well endowed with material resources. Using novel data on 880 national associations, gained from a survey of interest groups in five European countries, we find support for these hypotheses. The article has implications for the literatures on lobbying, Europeanization, and theories of European integration.