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In this paper, we explore the reasons behind the limited presence of psychologists in addressing homelessness in France, despite the well-documented mental health needs. We examine how the social sciences have contributed to advancements in supporting people experiencing homelessness, contrasting with the historical lack of research from psychology on this issue. We also discuss the compartmentalization between the medical and social sectors, creating barriers to mental health care accessibility and limiting psychological interventions. This article concludes by highlighting recent strides in the mental health of the most vulnerable populations and advocates for initiatives to adopt a comprehensive medico-psycho-social understanding of homelessness.