This qualitative study, built around non-directive interviews conducted in 2014 at the Montperrin Psychiatric Hospital in Aix-en-Provence, aims to document the issue of cannabis use in a closed institutional situation. Referring to the precepts of Karl Rogers for the conduct of research interviews, the author wanted to offer the freest and least distorting framework possible to the patients interviewed. This book, which is highly integrative in its approach and references, offers a comprehensive, cross-disciplinary look at the issues raised by cannabis use in closed institutions, at the personal, relational, group, systemic and institutional levels.