This chapter focuses on the psychiatric aspects of intellectual disability. Emerging research in this area has progressed from epidemiological studies to investigations into the causation of problem behaviours and comorbid mental disorders. The chapter describes the causation and the heterogeneity of dual disability. The presence of mild intellectual disability rarely has a single identifiable cause but is a consequence of both polygenic and social/environmental influences. The chapter deals with psychiatric and behaviour disorders in people with intellectual disabilities. In the case of both the presence of an additional mental disorder or of behaviour problems, a combination of developmental, biological, social or psychological factors interact and influence the occurrence of such behaviours. The chapter presents the prevention and detection of mental ill health and its treatment. It discusses the maintenance of mental health and prevention of mental ill health.