Background: Mental disorder and intellectual disability each accounts for substantial burden of disease. However, the extent of this co-occurrence varies substantially between reports. We sought to determine whether studies in children and/or adolescents with acceptably rigorous methods can be distinguished from existing reports, and whether key risk factors could be ascertained. Method: Published studies investigating the prevalence of mental disorders in children and/or adolescents with intellectual disability were reviewed. Results: Nine studies with acceptable methods were identified, 4 which compared the prevalence of mental disorder in populations of those with and without intellectual disability, and a further 5 studies that estimated the rates of mental disorder in those with intellectual disability were identified. Collectively, these studies demonstrate rates of comorbidity for children and adolescents between 30 and 50% with a relative risk of mental disorder associated with intellectual disability ranging from 2.8-4.5. The risks for this comorbidity associated with age, gender, severity of intellectual disability, and socioeconomic status remain uncertain. Conclusions: Appreciation of this comorbidity needs to be a fundamental component of both mental health and intellectual disability services. (Contains 2 tables.)