The research presented here is justified by the lack of tradition in Spain with regards to socio-educational intervention programmes based on training and group support, as well as the proven benefits of these methods in foster care. It addresses the need to go deeper into the acquisition and consolidation of the core professional competences for running socio-educational groups with foster families. After a process of participatory action-research with 39 professionals from the Child Protection System from different regions of Spain, we see an improvement in the development of the competences necessary to establish a supportive relationship with families, management and group dynamics, as well as the crystallization of attitudinal changes in professional practices. Data have been collected on the basis of methodological complementarity and a pre-test–post-test design that has identified and explained the changes in the evolution of competences and professional practices.