The main objective of SUMHIT is to examine and assess the place of people who use drugs in the field of mental health care, the availability of generic mental health care for people with substance-use disorders, and the capacity of both generic mental health care and specialised substance-use care sectors to collaborate within the framework of the service networks that have been established in Belgium since 2010. The project addresses research questions at three levels of study. At the level of individuals, it assesses the met and unmet needs of people with substance-use disorders in terms of mental health care, their actual access and use of services, both generic and specialised, and their lived experience of care pathways. The proposal includes a focus on specific sub-profiles, such as gender and age groups, or specific psychosocial and medical vulnerabilities. At the level of services, it assesses the experience of clinicians from the full range of service types included in mental health networks and from specialised substance-oriented services, in particular with people who have substance-use disorders and other mental health care needs. It also examines the availability and accessibility of care supply for such patient profiles, and the existing (good) practice of collaboration between the specialised substance-use sector and the generic mental health care sector. Finally, at the level of service networks and whole care system, the project assesses the collaboration / integration of specialised SUD services in the generic mental health care networks established by the ‘Title 107’ reform and reviews systemic mechanisms (in terms of funding, service provision, and governance) that may facilitate or hinder such collaboration.