During recent years there has been greater recognition of the impressive degree of psychiatric disturbance that affects those with mental retardation. An increasing number of systematic studies are focusing on the prevalence, risk factors, and clinical characteristics of psychiatric disturbance within this population. In addition, traditional neuropsychiatric assessment and treatment approaches are being evaluated, and new approaches are being developed. Such efforts will undoubtedly improve the ability to effectively diagnose and treat mentally retarded individuals suffering from psychiatric disorders. This paper is the second of two reviews that explore several recent developments in biological, phenomenological, and psychopathological aspects of mental retardation.