People with severe mental illness and co-occurring substance use disorders, also referred to as dual disorders, experience worse outcomes over the long term than people without co-occurring substance abuse. Integrated treatment of both disorders has been shown to be more effective than separate treatments offered in parallel or in sequence. The principles and strategies of integrated dual disorder treatment (IDDT) include integration of treatments for the mental illness and the addiction, use of strategies to engage people in treatment, use of pharmacologic and psychosocial interventions that are matched to the patient's stage of change, and use of a long-term perspective. The stages of change, the stages of treatment, and the psychosocial strategies used at each stage of treatment are outlined.