The definition of educational alliance as a heuristic and design device is the outcome of a Participatory Action Research conducted within the Prin Re-seves, Research at the Service of Educational Fragilities, in which the Naples Research Unit – coordinated by Prof. Pascal Perillo – focused on the representation that teachers and educators have of adolescents’ antisocial behavior. During the third year, with the aim of building a network system on the territory, six territorial educational alliances were formed composed of schools and educational agencies that worked together to design an educational intervention intended to prevent adolescents’ antisocial behavior. The most relevant aspects of the co-design work were: the participatory production of knowledge; the strengthening or creation of collaborative networks; the continuity between formal, nonformal and informal; and the possibility of considering the recipient of the interventions in different life situations. The educational alliance is thus proposed as a space for professional training, reflection and design of complex interventions.