Estimates of the prevalence of comorbidity of psychiatric disorders and mental retardation in community and clinical populations range from 14.3 to 67.3 percent. A wide variety of disorders have been reported in this population, including schizophrenia, depression, and, commonly, conduct disorder. The incidence of specific disorders appears to be related to the level of retardation and the concomitant presence of seizure disorder. Accurate assessment of psychiatric disorders in this population is difficult because mentally retarded patients have poor communication skills and because most diagnostic instruments were developed for persons of normal intellectual functioning. Treatment includes educational, behavioral, and pharmacological interventions, but guidelines for safe use of psychotherapeutic drugs are needed.