1. Normal joints in individuals of all ages may tolerate prolonged and vigorous exercise without adverse consequences or accelerated development of OA. 2. Individuals who have underlying muscle weakness or imbalance, neurological abnormalities, anatomical variances, and who engage in significant amounts of exercise that stress the lower extremities, may accelerate the development of OA. 3. Individuals who have suffered injuries to supporting structures may also be susceptible to accelerated development of OA in weight-bearing joints, even without increased stress to the joint from exercise. 4. Certain individuals with established degenerative or inflammatory arthritis may benefit from supervised exercise programmes. 5. Still more information is needed so that physicians can identify subjects at risk for the development of OA, advise the millions of participants about the beneficial and deleterious effects of regular exercise and sports participation, and develop successful rehabilitation programmes for injured joints.