Europeanization is understood as domestic adaptation to European integration. This chapter outlines the literature on party Europeanization, and highlights historical differences in challenger and mainstream parties’ interactions with Europe. It presents data and methods, followed by the analysis that addresses organizational Europeanization before moving on to the party programs. Through qualitative analysis of Belgian party statutes and manifestos, the chapter shows how challenger and mainstream parties show different patterns to Europeanize in different ways, the former showing more programmatic and the latter more organizational Europeanization. It finds patterns of Europeanization specific to party family and ideology and to the particular context of Belgium. When it comes to programmatic Europeanization, one can follow the literature in the expectation that challenger parties will be more likely to politicize the European Union. To identify programmatic Europeanization, election manifestos are the most adequate source of information.