The epidemiology of disability uses epidemiological methods to describe the disability process and to identify factors that improve or delay its course. A conceptual model for the way chronic health problems affect functional ability is examined with an emphasis on endurance and muscle strength and examples from a Danish cohort study. Disability is difficulty in doing complex activities because of a long‐term physical or mental condition. In Glostrup only 10% and 18% of 75‐year‐old men and women, respectively, were dependent on personal help to perform mobility functions such as moving or walking outdoors, but 66% of the men and 71% of the women became tired during some or all of these activities. Functional limitations are restrictions in doing basic physical and mental activities. Up to 50% of 75‐year‐olds with loss of the ability to step a height of 40 cm or‐walk at least 1.4 m/s may have difficulties in city traffic. Impairment refers to dysfunctions in specific body systems. Muscle strength is reduced in the elderly by 10% per decade. The 75‐year‐olds who performed mobility functions without tiredness had 50% higher muscle strength values in several muscle groups and 15% higher VO2max. The pathological conditions most commonly associated with functional disability in old age are in the locomotor and cardiovascular organ systems. The influence of living conditions and individual lifestyle on functional ability is mainly indirect through the influence of those factors on chronic morbidity. Smoking and low physical activity are, however, direct risk factors. At all levels of tie disability process, it is possible by regular physical activity and exercise to prevent further consequences of disease. Focused intervention strategies are needed during the total life course. In 75‐year‐olds, light physical activity 4 h per week is sufficient. Exercise in impaired but only moderately disabled people is perhaps the most cost‐effective effort against the burden of disability facing society with the