The term "duplication" should be used when describing those spherical or tubular structures found along the alimentary tract whose walls contain well-developed muscular layers and whose lining is a mucosa similar to that of some part of the alimentary tract. They should not be confused with mesenteric lymphatic cysts, or anomalies arising from a Meckel's diverticulum. Symptoms produced by these lesions usually vary according to their location, their size, and the type of fluid secreted by their mucosa. Duplications should almost invariably be removed, even though they are incidentally discovered and have not as yet given rise to symptoms. The pitfalls met with surgical treatment of these lesions are discussed. In this series, cases treated prior to 1940 carried fatality rates which were quite high; in the 20 cases treated in the last four years there have been no deaths. Sixty-eight examples of duplications from one institution are briefly listed; 45 of these have not appeared in the literature before. This amount of material coming from one hospital would seem to indicate that these abnormalities must occur with sufficient frequency to warrant the attention of pediatrician, roentgenologist, and surgeon so that they can be recognized promptly and treated suitably.