Computed tomography (CT) has provided a valuable imaging modality for the study of renal masses. Simple cysts are the most common of renal masses, and they are usually readily differentiated by CT from the wide variety of solid tumors that affect the kidneys. This article describes the CT technique which is used for evaluation of renal masses; the CT appearances of such renal tumors as renal cell carcinoma, renal pelvic carcinoma, lymphoma, oncocytoma, and angiomyolipoma and the use of CT in the staging of malignant renal tumors. The CT findings in various renal cystic diseases are also described. The role of CT for evaluating renal masses in relation to such other imaging techniques as sonography and angiography is discussed.