Evidence from the US shows that integrated treatment programmes for dually diagnosed patients are more successful than parallel treatment programmes. In the UK the Dual Diagnosis Good Practice Guide (DDGPG, 2002a), advocates a move towards an integrated system of care delivery. However, the paucity of evidence in the UK and the entrenched nature of the established mental health and addictions services means that current policy is derived from limited information and is struggling to address the process of change. By definition, dual diagnosis is a complex interaction between a range of mental health and substance misuse problems leading to difficulties in allocating appropriate skill mixes to teams. Ethical and legal issues in the mental health services cause conflict with the treatment concepts for substance misuse. The advent of the DDGPG is positive, but there is a clear need for further work in this area.