In high-income countries, about 5/1000 adults have intellectual disabilities. Population prevalence of intellectual disabilities is higher in children/young persons than adults, and prevalence decreases in older age groups to about 2/1000 over the age of 65. Intellectual disabilities are more common in boys/men, and in low-income countries; prevalence varies with geography and over time. Mental ill health is more common in people with intellectual disabilities than in the general population, with a point prevalence of 41% in adults and 36% in children/young people. Physical health problems and disabilities are common, and multi-morbidity and polypharmacy typical in people with intellectual disabilities; hence, clinical assessment and management are complex. The lifespan of people with intellectual disabilities is currently about 20 years less than that of other people, and more than 37% of their deaths are preventable deaths amenable to high-quality health care. Improving health care for people with intellectual disabilities needs to become a priority for clinicians, service commissioners, and policymakers.