Athletes, particularly those who are involved in sporting activities requiring repetitive overhead use of the arm (for example, tennis players, swimmers, baseball pitchers, and quarterbacks), may develop a painful shoulder. This is often due to impingement in the vulnerable avascular region of the supraspinatus and biceps tendons. With the passage of time, degeneration and tears of the rotator cuff may result. Pathologically the syndrome has been classified into Stage I (edema and hemorrhage), Stage II (fibrosis and tendonitis), and Stage III (tendon degeneration, bony changes, and tendon ruptures). The impingement syndrome may be a problem for the young, active, and competitive athlete as well as the casual weekend athlete. The "impingement sign" which reproduces pain and resulting facial expression when the arm is forceably forward flexed (jamming the greater tuberosity against the anteroinferior surface of the acromion) is the most reliable physical sign in establishing the diagnosis. Flexibility exercises, strengthening programs, and special training techniques are a preventive and treatment requirement. Rest and local modalities such as ice, ultrasound, and antiinflammatory agents are usually effective to lessen the inflammatory reaction. Surgical decompression by resecting the coracoacromial ligament or a more definitive anterior acromioplasty may rarely be indicated.