One of the major puzzles for contemporary European political science is how and to what extent the process of European integration has affected national politics, not only in terms of Europeanization of national policies and institutions, but also in terms of effects on electoral campaigns, mass voting behaviour, political attitudes and changes in the political supply (Goetz & Hix, 2012). From the supply side, national elites can see European integration as a challenge or an opportunity. In particular, the ways in which elites, and specifically political parties either in government or in opposition, position themselves on the issue of European integration has been broadly explored by the literature (Marks et al., 2002; Hutter et al., 2016). However, the literature is not conclusive on the main determinants of party positions on European integration, nor on the explanations of voters’ attitudes towards the EU. The response of a political party to an issue emerging on the national agenda or acquiring new salience can be determined by a large set of predictors, ranging from party leaders’ rationalities, to the format and competition structure of the national party system, to the constraints posed by prior policy positions or the territorial structure of the state (Steenbergen & Scott, 2004). This chapter therefore tries to disentangle the interplay between meso- and macrolevel variables affecting the attitudes towards the EU of national elites and citizens in a case study, namely Belgium.