Aortic aneurysm is the 13th leading cause of mortality in the United States. Thoracoabdominal aortic aneurysm (TAAA), defined as straddling the diaphragm and extending to a point anywhere between the left subclavian artery and the aortic bifurcation, remains a less common entity than aneurysm involving the ascending or infrarenal aorta. In 1986, E. Stanley Crawford popularized a classification scheme for TAAAs based on their extent, a surrogate for the risk of postoperative neurological deficit. This system essentially classifies the extent of aneurysm as primarily involving the thoracic aorta with varying degrees of extension into the abdominal aorta (types I and II) or as primarily involving the abdominal aorta with varying degrees of extension into the thoracic aorta (types III and IV). The natural history of aortic aneurysm remains one of inexorable expansion and rupture; classic early studies of untreated TAAA revealed a 2-year mortality of 76%, predominantly from aortic rupture. The strongest determinants of the risk for rupture are aneurysmal diameter and growth rate. Other predictors of rupture include advanced age, a history of hypertension, tobacco use, and chronic obstructive pulmonary disease. Patients presenting with larger aneurysms or superimposed dissection typically have higher growth rates and are at greater risk for rupture. This chapter discusses the indications for intervention as well as operative approaches, both open and endovascular.