This article focuses on the design of a participatory diagnosis within a participatory action research (IAP) framework. The first part discusses the theoretical aspects of IAP, emphasizing its focus on social change and transformation. It highlights the basic characteristics, key elements, the importance of the motor team, and the structural phases of IAP. The second part presents a practical example of a participatory diagnosis conducted in three secondary schools in L'Hospitalet de Llobregat, Barcelona, Spain. The study aimed to contextualize the territory and identify the channels and level of participation among young people. The theoretical framework and the practical example together allow for reflection on the integration of the three defining elements of IAP: research, action, and participation. This approach involves a rethinking of epistemology and methodology, based on a critical analysis of active community participation to encourage transformative action and social change.This article focuses on the design of a participatory diagnosis within a participatory action research (IAP) framework. The first part discusses the theoretical aspects of IAP, emphasizing its focus on social change and transformation. It highlights the basic characteristics, key elements, the importance of the motor team, and the structural phases of IAP. The second part presents a practical example of a participatory diagnosis conducted in three secondary schools in L'Hospitalet de Llobregat, Barcelona, Spain. The study aimed to contextualize the territory and identify the channels and level of participation among young people. The theoretical framework and the practical example together allow for reflection on the integration of the three defining elements of IAP: research, action, and participation. This approach involves a rethinking of epistemology and methodology, based on a critical analysis of active community participation to encourage transformative action and social change.