A thoraco-abdominal aortic aneurysm is defined by dilatation of the aorta to a diameter that is at least 50%greater than the expected normal diameter at the diaphragmatic hiatus, with varying degrees of thoracic and abdominal extension. Because thoraco-abdominal aortic diameter varies from its most proximal section (just distal to the left subclavian artery) to its most distal section (which includes varying sections of the abdominal aorta), one must assess aortic diameter relative to nearby healthy sections of aortic tissue while accounting for the effects of age, sex, and body size. Thoraco-abdominal aortic aneurysms should be interpreted in the context of their causes, the two most common of which are medial degeneration and aortic dissection. Aneurysms caused by aortic dissection can be further classified in terms of the extent of aortic involvement, acuity, and type of aortic wall disruption. For more than three decades, thoraco-abdominal aortic aneury sm repairs have been classified according to the extent of aortic replacement; the Crawford classification system facilitates risk stratification, helps surgeons plan the surgical approach and select protective adjuncts, and facilitates standardized reporting of results.