At present the European Union is confronted with one of the biggest economic crises in history: Unemployment and poverty rates are on the rise, while social security systems and labour-law related institutions are increasingly dismantled (especially in the so-called "crisis countries"). Thus, the implementation of a "social Europe" seems far out of reach. Against this background, the PhD thesis deals with the question, if and how trade unions can contribute to (the fight for) a more social Europe. Theoretically, the work is based on the political writings of the French sociologist Pierre Bourdieu on the one hand, in which he argues that unions need to be more offensive and include their members in their (political) actions to be able to organise transnational countervailing power. On the other hand, the thesis takes into account Pierre Bourdieu's field theoretical approach, which helps to analyse with which struggles, power imbalances and dominant doxa unions are confronted in their attempts to intervene in (national and supranational) political fields. Furthermore, field theoretical approaches draw attention to the social processes between trade unions in Europe, which are also influenced by power asymmetries as well as social struggles over material and symbolic resources. Empirically the thesis is based on the (historical) analysis of the European Action Days. These Action Days are cross-border trade union mobilizations mainly taking place on the political (rather than on the enterprise or on the sectoral) level. The Action Days are analysed from a transnational and a comparative perspective. Four country reports (based on qualitative interviews and secondary analysis of existing research) focus on the perspectives of trade unionists from different member states (Austria, Bulgaria, Finland, Spain). A quantitative survey with a questionnaire sent to all member organisation of the European Trade Union Confederation (ETUC) as well as an analysis of press releases and mobilization posters were conducted to get more information on unions' participation rates in the Action Days. The thesis shows that unions' participation in the Action Days depends on different economic, political and social factors, e.g. on the unions' position within the (European and national) political fields as well as on the willingness of politicians to include unions' interests in democratic decision making processes. Moreover, the establishment of a shared transnational framing of the situation, protest experiences and memories as well as different trade union traditions and power resources play an important role.