Five cases of mastoid surgery in which Korner's septum was identified have been presented. Identification of this structure insured complete exenteration of the mastoid and thus reduced the tendency of recurrent mastoid disease. The clinical significance of the duality of the antrum and the mastoid process in its relationship to disease of the middle ear and mastoid is clarified by an understanding of Korner's septum. The relationship of the parts of the temporal bone in an infant and adult skull and the process of pneumatization of the mastoid portion of the temporal bone have been reviewed. A thorough knowledge of the surgical anatomy of the temporal bone is the key for satisfactory mastoid and middle ear surgery.