A relatively large and rare group of primary tumors of the jaw arising from the embryologic structures of the tooth or remnants of the tooth germ have been known for many years, but, as with many other conditions in medicine, they are not fully understood. These are of primary interest to the dental profession but should also be of interest to radiologists if not to the entire medical profession. The tooth follicle from which a tooth develops is made up of both epithelial and mesoblastic tissue. The enamel organ develops from an epithelial cord originating in the mucous membrane of the mouth, while the papilla, which forms the tooth pulp, cementum, and dentine, comes from the mesoblastic tissue growing into the invaginated enamel organ. These structures are, in turn, surrounded by a fibrous sheath known as the tooth sac. All of the tumors under discussion arise from one or more elements making up the tooth follicle. There is considerable confusion both as to classification and identification. Some writers contend that the cystic types should not be classed with the solid and calcified tumors. The most widely accepted classification is that submitted by a special committee of the British Dental Association (1), which includes all types. This would also appear to be preferred from the radiologic viewpoint. Every primary tumor of the jaw arising from one or more of the dental formative organs, therefore, becomes an “odontoma,” a word derived from two Greek words meaning tooth and tumor. 1. Epithelial Odontomas. (A) Multilocular cysts. (B) Dentigerous cysts. (C) Dental cysts. 2. Composite Odontomas. (A) Irregularly calcified tumors. 1. Complex composite odontomas. 2. Compound composite odontomas. (B) Tooth-like tumors. 1. Geminated composite odontomas. 2. Gestant composite odontomas. 3. Enamel nodules. (C) Dilated composite odontomas. 1. Dilated crown. 2. Dilated whole mass. 3. Dilated root. 3. Connective-tissue Odontomas. (A) Fibrous odontomas. (B) Cementomas. I. Epithelial odontomas, as the name indicates, are the result of an abnormal development in the epithelial elements alone. (A) Multilocular Cysts.—These have also been known as adamantinomas. They are seen twice as frequently in women as in men, and are but rarely seen in the maxilla as compared to the mandible. They appear most frequently between the ages of 20 and 35 years, being slow in growth and painless. The tumor is made up of separate epithelial-lined cysts of varying sizes and numbers. The septa may be purely fibrous or contain bone. The epithelial lining shows varying degrees of proliferation, sometimes producing an almost solid tumor. There are indications that at least part of these tumors originate from the gingival epithelium and some, at least, either undergo malignant change or are primarily malignant in that they have the appearance of basal-cell carcinomas and have been known to metastasize, especially after incomplete removal.