Inclusion health aims to prevent and address health and social inequalities for people experiencing exclusion, such as people experiencing homelessness, people who have a history of contact with the criminal justice system, people who use drugs, sex workers, vulnerable migrants, victims of modern slavery and human trafficking, and Romany Gypsy, Roma, and Travellers communities. These populations have poor health outcomes and disproportionate health inequities, partly resulting from inadequate health-care access. Outreach services can improve health-care access, but there is little evidence of how outreach operates successfully. We conducted a realist review of multicomponent outreach health-care services to understand the circumstances under which outreach works for people experiencing exclusion and why. Key components of effective outreach include person-centred services and appointments, staff expertise, high-quality communication, and close partnership with people experiencing exclusion and relevant organisations. Service users are likely to develop trust and further engage through positive experiences and regular interaction with the same staff.