Os odontoideum (OO) and absentia odontoidei (AO) are rare conditions, which mainly are of interest because they may cause atlanto-axial instability. Traditionally they are considered to be congenital lesions. However, during the latest decades a number of acquired cases have been reported. Presently it is generally believed that the two conditions may exist in both a congenital and an acquired version. A review of the literature about OO fails to disclose any solid evidence in support of the classical theory of congenitality. It is concluded that OO and AO are diseases of childhood and adolescence. They are probably always acquired, and result from avascular necrosis of the odontoid process during the growth process.