This report by the President's Council on Physical Fitness and Sports examines the effects of various forms of physical exercise on the knee joint which, because of its vulnerability, is especially subject to injury. Discussion centers around the physical characteristics of the joint, commonly used measurements for determining knee stability, muscular and ligament strength. The deep knee bend, an exercise of highly questionable value in conditioning regimens, is criticized on the grounds that it contributes to chronic synovitis, promotes early arthritic onset, and produces gross knee instability. Exercise regimens are proposed and discussed for the treatment and prevention of knee injuries. Practical advice for the development of physical education and athletic programs justifiable from this review of knee joint exercise is given, and includes: (1) Exercises that unduly stretch or damage the ligaments of the knee should be avoided; (2) The deep knee bend should be used sparingly, if at all, in physical education and athletic conditioning, and substitutes should be developed such as bench stepping and weight training routines; (3) In performing knee bends for developing and maintaining the strength and endurance of the quadriceps muscles, a half knee bend is recommended; (4) Progressive resistance exercises of the muscles activating the knee joint should be routinely employed for the prevention of knee injuries and for the rehabilitation of the knee during postinjury or postsurgical rehabilitation. A bibliography of 37 citations is appended. (MB)