Aortic stenosis, or narrowing of the aortic lumen, has many causes. It may originate in coarctation or pseudocoarctation of the aorta, midaortic dysplastic syndrome, atherosclerosis, Takayasu arteritis, aortic dissection, or various intraaortic and periaortic diseases or as a result of aortic surgical repair. The impedance of blood flow through the stenotic segment may lead to the development of various collateral arterial pathways, according to the location of stenosis. Aortography is the standard technique for evaluating aortic stenosis; however, helical computed tomography (CT), particularly multisection CT, may provide additional information or in some cases may be used instead of arteriography. Multisection CT can depict the aorta and thoracoabdominal collateral pathways in less than 1 minute and provide high-quality arterial-phase imaging data suitable for multiple two-dimensional and three-dimensional reformations. To produce a useful differential diagnosis, the imaging specialist must be able to recognize the type of stenosis and the configuration of collateral circulatory pathways.