This study analyses the evolution of social assistance in Italy, Spain, Portugal and Greece, and closely examines the four countries’ different experiences with Guaranteed Minimum Income (GMI) schemes. A process-tracing methodology uses data from secondary sources, archival material, and 46 interviews to construct an actor-centred model and pursue a multiple-causality, historical approach. Outcomes are shown to result from interactions among central governments, religious organizations, secular organizations and territorial actors; and also from destabilizing forces. It is assumed that social assistance beneficiaries are forced to rely on these actors, whose attitudes are found to vary significantly due to their different interests, subjective perceptions of fairness, and preferences. Case histories of the four countries show that the periods prior to the 1970s were marked by minimal central government interest; indifferent, hostile, and/or divided secular organizations; and governmental partnerships with religious organizations. In the post-1970s periods, destabilizing forces co-occurring with centre-left governments resulted in new policies and changes, with relevant actors/organizations gradually welcoming pluralistic social assistance systems. The existence and extent of GMI schemes has been the principal factor differentiating social assistance developments among the four countries in more recent decades: Portugal is the only country with a national GMI, Italy and Spain have solely regional schemes, and Greece has no GMI at all. Because GMIs cut across traditional social assistance categories and are often linked with overall welfare system restructuring, establishment of GMIs and their subsequent maintenance require the co-occurrence of destabilizing forces and strong pro-GMI coalitions. Portugal exhibits the highest level of pro-GMI consensus nationwide, Greece the lowest,while Italy and Spain occupy intermediate positions. The institutional empowerment of territorial actors in the latter two countries was a precondition to emergence of local schemes, while destabilizing forces and strong local pro-GMI coalitions greatly increased the odds for establishing and maintaining them.