Over the last several decades there has been an improved understanding of the intricate anatomy that provides stability to the glenohumeral joint. In addition, significant advances in identifying the pathologic etiology of the unstable shoulder have occurred because of basic science glenohumeral ligament cutting studies, clinical evaluation, and the advent of arthroscopic evaluation and treatment of the unstable shoulder. This article will review the pertinent anatomy of the normal glenohumeral joint and will carefully review the pathoanatomy found in the unstable shoulder. Sports medicine specialists who treat athletes with unstable shoulders should have a firm understanding of both the normal and pathologic shoulder conditions to be able to provide the best care for these athletes.