Attempts have been made to establish objective diagnostic criteria for psychiatric disorders in persons with mental retardation. This paper describes the clinical features of the most important psychiatric disorders in mentally retarded adolescents: mood disorders, psychotic disorders, severe behavioral disorders, personality disorders, anxiety disorders, and attention deficit disorder with hyperactivity. The impact of mental retardation on personality development is confirmed by the high psychopathological vulnerability of the mentally retarded. All types of mental disorders can be observed, with an incidence estimated to be at least three or four times higher than in the general population. Adolescence is a particularly important phase for the mentally retarded, since adolescent turmoil can increase the risk of psychopathology.