The co-existence of substance misuse and psychiatric disorders is often referred to by the term 'dual diagnosis'. The health-related problems associated with dual diagnosis has received growing attention by clinicians and researchers alike. Severely mentally ill patients who misuse drugs (prescribed and illicit) or alcohol have complex health and social needs. Mental health nurses have a key role to play in the early recognition and management of this group of patients. This paper examines the nature and extent of dual diagnosis and its implications for mental health nurses. It also addresses treatment strategies that are required to meet the needs of this cohort of patients.