Comorbid substance abuse disorders have emerged as one of the greatest obstacles to the effective treatment of persons with schizophrenia. Estimates of the prevalence of such comorbidity vary, but as many as half of persons with schizophrenia may suffer from a comorbid drug or alcohol disorder. Younger age, male gender, and lower educational attainment are associated with greater risk for addiction. Persons with schizophrenia and comorbid addiction tend to have an earlier onset of schizophrenia than do those without comorbid addiction. Research does not support a link between specific symptoms of schizophrenia and choice of abused drugs. Rather, drug choice is correlated with the pattern of ambient drug use in the community. Comorbid substance disorders are associated with a variety of poorer outcomes, including increased psychotic symptoms, poorer treatment compliance, violence, housing instability and homelessness, medical problems (including human immunodeficiency virus infection), poor money management, and greater use of crisis-oriented services that result in higher costs of care. Considerable progress has been made over the past decade in understanding the need to integrate substance abuse treatment and mental health treatment to provide more effective care for this population.