Mental retardation (MR) is a heterogeneous condition defined by significantly subaverage intellectual and adaptive functioning and onset before age 18 years. With an approach underscored by principles of normalization and the availability of appropriate education and habilitation, persons with MR generally live, are educated, and work in the community. Mental disorders occur more commonly in persons with MR than in the general population. However, the disorders themselves are essentially the same. Clinical presentations can be modified by poor language skills and by life circumstances, so a diagnosis might hinge more heavily on observable behavioral symptoms. The diagnostic assessment considers and synthesizes the biological, psychological, and psychosocial context of mental disorders. Comprehensive treatment integrating various approaches, including family counseling, pharmacological, educational, habilitative, and milieu interventions is the rule.