Purpose -There is ongoing discussion around how to structure psychiatric services to meet the needs of people with intellectual disability and co-morbid mental illness and several different models have been suggested. With research evidence lacking, there is a lack of consensus as to the best model of service provision. This paper aims to review the current knowledge in this area and discuss the salient issues.Design/methodology/approach -This is a review article summarising the current debate. Evidence from original research is presented and combined with opinion from clinical experience.Findings -The authors find a lack of robust research evidence to support any particular model of service provision. However, it seems to be increasingly accepted that purely generic models of care for people with intellectual disabilities and co-morbid mental illness are not appropriate. Integration of the expertise from specialist services within mainstream services is presented as potentially the most advantageous approach.Originality/value -This article will be of benefit to clinicians working in the field of intellectual disability and mental illness, and to managers and commissioners with responsibility for providing services. It will also be useful for those new to the discipline to gain an overview of the current issues and debate within service planning for this population.