This opening chapter has two aims. First, it seeks to deepen the theoretical framework of this book by looking briefly at some of the main avenues of research on religion and politics: religion as a firm, religion as an organization, religion as an institution. In the light of this overview of the theoretical controversies surrounding our object of study, we propose to grasp the socio-political role of Social Catholicism on the ground in Southern Europe through an approach which derives both from Historical Institutionalism and from cognitive and relational approaches to social movements. Second, the chapter sets out the empirical material and research methodology used in this work. The general hypothesis will be tested in several subnational fields (Brescia and Forlì in Italy, the Basque Autonomous Community in Spain, Aquitaine in France), all located in Southern European countries with a Catholic matrix. In so doing, the chapter outlines the research design which informed this investigation into the politics of Catholic mediations.