The atlas and axis support the head on the lower cervical spine while providing for considerable mobility in flexion, extension, rotation and lateral bending. The first two vertebrae also function as conduits for the cervical cord and vertebral arteries. Lesions of the atlas and axis, therefore, can cause instability with loss of support and encroachment on the upper cord and vertebral arteries, or less often stiffness with restricted motion. Congenital lesions of the occipito-cervical spine such as occipitalization of the atlas or accessory occipital vertebrae can constrict the upper cord with osseous, dural, or fibrous compression. Such encroachment on the cord at this level produces a varying array of clinical complaints and findings frequently difficult to interpret. Instability of the upper cervical spine can result from congenital, traumatic, inflammatory or neoplastic disruptions of the interlocking mechanism of the atlas and axis. Such factors as the loss of structural integrity of the dens or stretching or tearing of the transverse ligament can permit instability with cord involvement. Because of the serious potential of these lesions, patients with abnormalities of the atlas and axis require prompt recognition and treatment.