Data on the natural history of descending and thoracoabdominal aneurysms are limited to a few studies. They demonstrated that, independent from the different mechanisms of injury and degeneration affecting the structural integrity of the aortic wall, the biologic fate of all aneurysms is progressive enlargement and rupture. Laminated thrombus and calcification do not prevent this process. The natural history is markedly influenced by size, location, symptoms, and etiology of thoracic aneurysms. They may develop symptoms related to mechanical compression of adjacent structures, but more frequently they are asymptomatic until rupture occurs. Usually, aortic rupture causes death by exsanguination and shock before bleeding can be controlled. Occasionally, rupture can be contained by the parietal pleura or occur towards the mediastinum, esophagus, pulmonary parenchyma, or bronchi. Untreated aneurysms will cause death in the majority of patients because of rupture. Recent data evaluating smaller and asymptomatic thoracic aneurysms with CT scan support nonoperative management and close follow-up of descending and thoracoabdominal aortic aneurysms only when the diameter is less than 5 cm. Patients with Marfan syndrome may be operated upon with smaller aneurysms. Independent of etiology, operative repair is the most effective means to alter the otherwise malignant course of descending and thoracoabdominal aortic aneurysms. The role of endovascular repair of these aneurysms remains to be established; however, it may represent an alternative to treat thoracic aneurysmal disease in very high risk patients.