This briefing examines the issues presented by service users with dual diagnosis for UK practitioners in health and social care. Confusingly, the term ‘dual diagnosis’ is used to describe several combinations of physical, psychological or developmental conditions; but for the purpose of this briefing, it refers to the co-existence of substance misuse and mental health problems. This briefing considers all age groups and uses the term ‘substance’ to refer to illegal or illicit drugs; alcohol; nicotine and prescription drugs. The terms ‘substance’ and ‘drug’ are used interchangeably. ‘Mental health problems’ refers to severe or enduring conditions, while ‘substance misuse’ refers to chronic or complex substance use problems. The briefing does not consider specific pharmacological or other treatment interventions in detail, but focuses on issues arising at the health and social care interface. It draws on research and literature from other countries, including the US where the majority of research on dual diagnosis has been conducted; to provide an overview for health and social care practitioners in the UK. Where there are gaps in the research, for example, in regard to service user involvement, recovery approaches and personalisation of services, the briefing draws upon evidence from relevant fields such as mental health and substance misuse. Throughout this briefing the terms, patient, client, and service user are used interchangeably to reflect the different usages prevalent within different sectors of health and social care.