In some of Quebec’s unionized sectors, important initiatives have been deployed in order to prevent mental health problems in the workplace. For over three decades, visionary union representatives have built working peer assistance structures, won important legal struggles and have developed innovative approaches aiming to correct or improve work organization. As the neoliberal ideology and its related work organization principles contribute to intensify work, leading to a weakening of the workers psyche, and as the balance of power favors employers more and more, it seems like an opportune time to survey the experience of those visionary union representatives in order to get a better understanding of union actions regarding mental health in the workplace and its structuring process. This thesis studies Quebec’s union initiatives concerning mental health in the workplace and aiming to prevent and correct problems like psychological distress, burnout, harassment, depression, violence, work related suicide, etc. In order to achieve this objective, a mixed theoretical approach is used. On the one hand, a large philosophical perspective is adopted. Authors Thomas Hobbes, Adam Smith, Karl Marx and Hannah Arendt are referred to in order to forge a better understanding of human interactions and action. By extracting their respective views of the world, of action and of social link, an analytic model is built, which could tie the life experience of union representatives to these different views, therefore achieving a better understanding of the ideological basis on which they built up their action and how it had influenced their individual and collective history. On the other hand, the social experience theory (Dubet, 2009; 1994) is selected as a way to analyze more precisely the work of union representatives. This theory identifies three logics of action, all complementary and in tension with one another, with which all social actors must deal with: integration, strategy and subjectivation. The coexistence of these three logics means that the actor’s experience of the living world is not a simple reproduction mechanism, applying social determination learned from their predecessors. Actors are also subjects of their own experiences and capable of distancing themselves from their own socialization in order to understand their act; they take place in the world in a critic dialectic. This theory sheds a light on what eases or restrains individual and collective action in regard to mental health in the workplace and describes how union representatives mobilize themselves in order to respond to the many needs of their members. This qualitative research uses a life trajectory methodology (Rhéaume 2008; Bertaux 2006). Twenty union representatives describe the suffering work experiences (Dejours, 2008) lived by workers and present initiatives they deployed in order to help them. The situations described by the participants show how some work organization elements are linked to domination experiences (Martuccelli, 2004) and have impacts on mental health in the workplace: the collateral damage of productivism and hyperflexibility, occupational injuries, occupational diseases and horror situations in the workplace, pernicious workplace social dynamics and abusive uses of the judicial system. The study also demonstrates the extent to which union initiatives contribute to solve these problems and the key factors in that regard: interdependence, empowerment, social justice and struggle for dignity. Four categories have been forged to describe those initiatives: social link care with peer assistance, legal defense, collective agreement clauses and actions targeting the work organization. Finally, the thesis presents three union representative profiles: militance which tries to build an us in the workplace, professionalization which tries to have its utility and competence recognized, and the peer assistants which aims to develop an action centered on the self. Their meeting shows the development of a union praxis which promotes and protects dignity of work and workers.