In the past decade epidemiologic and clinical studies have described the prevalence and incidence of disability and have elucidated demographic characteristics, health behaviors, diseases, and physiologic impairments that are associated with increased risk of developing disability in old age. Mobility disability has emerged as a critical aspect of functioning. Mobility, defined as the ability to move independently from one point to another, is a critical part of maintaining independence and an essential attribute of quality of life. In those age 85 years and older, nearly 60 percent of women and 40 percent of men have difficulty walking two to three blocks. Older people who are mobile are more likely to remain in the community, have lower rates of subsequent morbidity and mortality, and have higher quality of life. Recent work in biomechanics, clinical geriatrics, epidemiology, and clinical trials has helped to improve our understanding of the decline in walking ability in old age and the influence of multiple intrinsic and extrinsic factors. However, a great deal is still not understood about how older persons progressively lose the ability to ambulate. The InChianti Study has the potential to make major new advances in our knowledge of progressive decline in ambulation in older persons.