Individuals with mental illness and addiction comprise at least half of the patients in most mental health treatment systems. This combination results in increased risk for frequent psychiatric relapses, poor medication compliance, violence, suicide, legal problems, and high utilization of the emergency room or inpatient services. Traditional mental health and addiction treatments have not adequately addressed these co-occurring disorders due to clinical interventions, programs, and system flaws that have not addressed the individual's needs. Integrated treatment requires both an understanding of mental illness and addiction and the means to integrate and modify the traditional treatment approaches in both the mental health and addiction treatment fields. There is strong evidence to support the efficacy and effectiveness of integrated treatment in this population. All mental health clinicians should become experienced and skilled in the core psychotherapy approaches to treating substance use disorders, including motivational enhancement therapy, relapse prevention (cognitive-behavioral therapy), and 12-step facilitation. In addition, integrated treatment includes integrating medications for both addiction and mental illness with the behavioral therapies and other psychosocial interventions. This article reviews the clinical intervention, program, and system components of integrated treatment and specific clinical interventions for this population.