People with intellectual disability (ID) have a high vulnerability to develop mental health problems. The prevalence of mental disorders is higher than in the general population and, in addition, adults with ID often show behavioral problems that imply a need for psychiatric psychotherapeutic care. In view of the impairments of intellectual functioning, impaired adaptive behavioral skills and physical illnesses, the needs of this target group are usually complex and require particular expertise. A number of specific assessment instruments are available for target group-specific diagnostics to collate the cognitive performance and emotional development as well as for psychopathological symptoms and behavioral disorders. To improve the accuracy of diagnosis in the ID population, existing alternatives to DSM or ICD should be applied especially to adults with moderate or severe ID. Guidelines for psychopharmacotherapy and adapted user guides for psychotherapeutic treatment provide support in the context of target group-specific treatment.