This PhD dissertation concerns the processes of Europeanization of French and German associations mobilizing around the asylum issue. It seeks to analyse how these associations have been evolving since the start of the harmonisation of European asylum policies in the beginning of the 1990s. It focuses on a panel of associations (23 in total) representing the French and German pro-asylum movements. Their process of Europeanization is analysed through the examination of three fundamental dynamics: the Europeanization of their networks (the construction of inter-associative linkages across Europe), discourses (the construction of framing-processes having a European dimension) and collective actions (the construction of mobilizations addressing the European institutions). This thesis is situated at the intersection between three domains of research: social movements, European integration and migration policies studies. It aims to analyse a process of Europeanization 'from below' (as it concerns civil society actors) and 'from the margin' (as it concerns issues linked to the definition of its borders). Relying on an approach that is comparative (the analysis of France and Germany as different fields of mobilization on the asylum issue) and dynamic (the progressive definition of the EU level as a new field of mobilization), it analyses the question of knowing if associations having differentiated profiles and/or coming from different national contexts tend to have differentiated paths and degrees of Europeanization. Through the use of the concept of field of European mobilizations (associations constructing Europeanized collective actions evolve in a particular social space, defined by specific resources, rules and issues at stake), it shows that the Europeanization of social movement organizations corresponds to a process of inclusion into - and exclusion from - a particular field: that of the organizations gravitating around European institutions since the beginning of the 1990s. It shows then that associations having differentiated profiles and coming from differentiated national contexts follow a process of Europeanization from below, contesting the definition of EU borders that power-holders let prevail. They evolve in distinct fields of European mobilizations and use different resources and strategies to mobilize against EU asylum policy. Two main sources are used: semi-structured interviews and associative publications. The data collected was analysed through different methods developed in social movements studies: frame analysis, network analysis, protest-event analysis.