This article presents an algorithmic approach to the evaluation of posterior mediastinal masses seen with computed tomography (CT). CT remains the study of choice, since it not only can be used to help confirm the presence of these masses, but it also helps define the (a) location and extent of the lesion, (b) adjacent organ involvement, or (c) vascular involvement. Causes of posterior mediastinal masses include esophageal lesions, congenital or acquired vascular lesions, foregut cysts, intrathoracic goiters, mediastinal pseudocysts, fat-containing tumors, adenopathy, neurogenic tumors, infectious spondylitis, and vertebral tumors. From the CT appearance of the lesion, one can often distinguish among the various masses and identify their origin and cause. This information enables patient triage and therapy to be expedited and, in most cases of posterior mediastinal masses, allows a correct diagnosis to be made solely on the basis of the CT examination.