Supportive relationships between service providers and service users have been identified as crucial in the process and outcomes of forensic psychiatric treatment. These relationships are however challenged due to the presence of both care and control. As legitimate power is of major importance in criminal justice settings, for example in prisons which share similar characteristics with forensic secure units, theories of legitimacy and procedural justice can possibly reconcile the tension between care and control present in relationships between forensic service providers and users. In previous research the added value of legitimacy and procedural justice to understand relationships between forensic service providers and users has already been demonstrated from the viewpoint from service users. Nevertheless, according to the recently developed dual model of legitimacy in the prison environment both the viewpoint of forensic service users and service providers should be taken into account when studying legitimacy in forensic secure units. The present qualitative study therefore aims to complement existing knowledge about legitimacy and procedural justice in forensic secure units. Via semi-structured interviews and participant observations in three medium secure units I will assess the nature of respectively 1) self-legitimacy of forensic service users and 2) interactions between forensic service providers and users and the influence of these interactions on (self-)legitimacy.