The anatomic nature of a double aortic arch is described. Reference is made to the history of the malformation. The clinical picture and radiographic findings described by various authors are discussed. Two cases are reported which demonstrate two types of double aortic arch, One, a relatively common type, had a large right arch with a retro-esophageal component and a left descending aorta; the second, a rare type, had a large left arch with a retro-esophageal component and a right descending aorta. Further, one was an infant who had symptoms of constriction whereas the other was an adult who had no symptoms referable to the double aortic arch. Analysis is made of these cases and 47 other cases reported in the literature. This analysis reveals that: 1. There is a striking correlation between the occurrence of symptoms and the age and duration of life, but there is no correlation between the occurrence of symptoms and the type of double aortic. 2. A double aortic arch in which both components are patent throughout is more likely to produce symptoms than a double aortic arch in which one component is partially obliterated. The process of obliteration does not appear to give rise to symptoms. 3. There is considerable variation in the relative sizes of the components of a double aortic arch. The right component is usually the larger; part of the left component may be obliterated. To date, no case has been reported of obliteration of part of a persistent right Component. 4. A double aortic arch occurs more commonly with a left descending aorta than with a right descending aorta. 5. It is unusual for a double aortic arch to be associated with a malformation of the heart. The association of a double aortic arch with other vascular abnormalities is less rare. The two principal problems raised by the demonstration of a retro-esophageal vessel are discussed. The first concerns the nature of the malformation; the second concerns the advisability of operation in the given individual.