The co-occurrence of a severe mental illness and a substance abuse or dependence disorder is common enough to be considered the expectation more than the exception. Substance use disorders can occur at any phase of mental illness. 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, poor health, poor self-care, increased risk of suicide or risky behavior, and even possible incarceration. All of these factors contribute to increased burden and reduced capacity of the health care system to adequately treat patients. Screening, assessment, and integrated treatment plans for dual diagnosis to address both the substance use disorder and the mental illness are strongly recommended.