This report provides a rapid review of the literature on co-occurring substance use and mental health concerns and the integration of mental health and substance use services in Scotland. In doing so, it outlines evidence on the estimated prevalence of co-occurring substance use and mental health concerns, the barriers people experience in accessing services and support, and policy development to date. Finally, it identifies key recommendations highlighted in the literature with regard to improving services for people with co-occurring substance use and mental health concerns and highlights a number of key areas for further research.