Evolutionary imperfections render man's shoulder uniquely vulnerable to damage by wear and tear and by trauma. Impingement of the rotator cuff against the tip of the acromion leads to bursitis, attrition and tearing. The instability of the shoulder predisposes to dislocation and subluxation. Awareness of these imperfections can help the physician to localize the source of pain in the shoulder and to give rational and effective treatment.