The adverse effects of osteoarthritis on the quality of life for tens of millions of people and on the costs of health care and economic productivity make it a major health problem that will increase in incidence and impact with the aging of the population. Although in most patients the cause of osteoarthritis remains unknown and no cure has been identified, appropriate diagnosis and treatment, including education of the patients, can minimize symptoms and help patients maintain active and productive lives. Accomplishing this aim requires that physicians have an understanding of the pathophysiology of joint degeneration and the relationships between joint degeneration and the clinical syndrome of osteoarthritis. Although much remains to be learned about the development and progression of the joint degeneration responsible for osteoarthritis, it is clear that the loss of articular cartilage results from disruption of the structural integrity of the articular cartilage coupled with or caused by an imbalance in the anabolic and catabolic activity of the tissue. The progression of joint degeneration varies considerably among patients. In some individuals the joint degenerates rapidly; in others the degenerative changes progress slowly over decades and in others the disease may remain stable. In rare patients, the joint degenerative changes spontaneously improve. Although joint degeneration is the underlying cause of the symptoms of osteoarthritis, including joint pain and loss of joint function, not all patients with joint degeneration have symptoms of osteoarthritis. Future therapeutic approaches may include disease-modifying drugs and surgical procedures that correct mechanical abnormalities, debride joints, and replace degenerated articular cartilage with implants that stimulate restoration of a cartilaginous joint surface. Development of methods to detect and monitor subtle changes in cartilage metabolism and identify the joint changes that precede loss of articular cartilage may make it possible to detect the earliest signs of osteoarthritis, when therapeutic interventions have the greatest potential for preventing progression of the disease.