The recognition of a variety of extraluminal gas collections can be of great value in the diagnosis of many diseases of the gastrointestinal tract. Several examples of gas formation in the walls of hollow gastrointestinal viscera are illustrated and discussed. Extramural gas collections in the retroperitoneum and mediastinum, and some unusual intraperitoneal collections are also illustrated and their probable etiologic mechanisms explained. It is hoped that an increased awareness of this group of important conditions will result in a more effective search for and recognition of these gas accumulations which are so characteristic in appearance and of so much prognostic importance.