Impingement syndrome of the shoulder may be produced by compression of the subacromial structures against the coracoacromial arch. The anterior tip of the acromion, the coracoacromial ligament, and the coracoid process have been implicated as sources of impingement. Anatomic specimens were examined to determine the anatomic sites on the coracoacromial arch that result in compression of the subacromial structures during specific shoulder motions that are known to produce impingement pain. The relationships of the supraspinatus tendon, the biceps tendon, and the greater tuberosity of the humerus with the coracoacromial arch were demonstrated through various arcs of shoulder motion in autopsy subjects. Biceps tendon impingement occurred predominantly against the lateral free edge of the coracoacromial ligament. Impingement of the supraspinatus tendon and greater tuberosity was demonstrated primarily against the acromial end of the coracoacromial ligament and the anterior tip of the acromion during arcs of flexion and internal rotation. The coracoacromial ligament was stretched by the greater tuberosity passing beneath it. This may explain the formation of traction osteophytes on the anterior acromion in patients with chronic impingement symptoms.