Osteoarthritis (OA), the most common form of arthritis, is a major contributor to functional impairment and reduced independence in older adults. OA of the hip and knee are two of the most important causes of pain and physical disability in community-dwelling adults. Symptomatic hand OA is a common disease among the elderly, and impairs hand function, this impairment being largely mediated by pain. Like other chronic diseases, the etiology of OA is multifactorial, and several local and systemic risk factors have been identified. Differences in the prevalence of OA may be attributable to both genetic and life-style factors. Disease definition may be based upon clinical or radiographic criteria, although case definition should rely on radiographic features for epidemiological studies. This review focuses on the functional impact of the disease, describes geographic differences in prevalence rates, discusses disease definition criteria, and summarizes the most common risk factors, including age, associated with the risk of OA.