Osteoarthritis, the clinical syndrome of joint pain and dysfunction caused by joint degeneration, affects more people than any other joint disease. There are no consistently effective methods for preventing osteoarthritis or slowing its progression, and symptomatic treatments provide limited benefit for many patients. Osteoarthritis disables about 10% of people who are older than 60 years, compromises the quality of life of more than 20 million Americans, and costs the United States economy more than $60 billion per year. The incidence of osteoarthritis rises precipitously with age; as a result, the prevalence and burden of this disorder is increasing rapidly. Study of the patterns osteoarthritis incidence and prevalence shows that it occurs frequently in the hand, foot, knee, spine and hip, but rarely in the ankle, wrist, elbow, and shoulder, and the most important universal risk factors are age, excessive joint loading, and joint injury. Analysis of the impact of osteoarthritis raises questions that include: Why does the incidence increase progressively with age? Why are some joints rarely affected? How do mechanical forces cause joint degeneration? What biologic and mechanical factors slow or accelerate the rate of joint degeneration? Answering these questions could lead to effective methods of preventing osteoarthritis and slowing its progression.