In this 2nd article in the series on older adults (persons aged > or = 65 years), the role of physical activity in preventing disability associated with aging in the absence of specific illnesses is discussed, and different types of activities and the measures of outcome are described. Evidence for physical activity as a prevention measure is restricted to primary and secondary prevention because the application of physical activity for tertiary prevention of disability is limited. Chronic disease is considered in the context of its influence on disability only when physical activity offers some potential benefit to elderly persons.