The co-occurrence of a severe mental illness and a substance use or abuse disorder is common in the United States as well as internationally and could be considered as more the expectation than the exception when assessing patients with serious mental illness. Substance use disorders can occur at any phase of the mental illness, perhaps even inducing psychosis. Causes of this comorbidity may include self-medication, genetic vulnerability, environment or lifestyle, underlying shared origins, and/or a common neural substrate. The consequences of dual diagnosis include poor medication compliance, physical comorbidities and poor health, poor self-care, increased suicide risk or aggression, increased sexual behavior, and possible incarceration. All of these factors contribute to a greater health burden, which reduces the health care system's capacity to adequately treat patients. Therefore, screening, assessment, and integrated treatment plans for dual diagnosis that can address both the addiction disorder and the mental illness are recommended in order to provide accurate treatment, after-care, and other health care to accommodate patients' social and vocational needs.