This monograph deals primarily with the kidneys, ureters, and urinary blad­ der. The kidneys are retroperitoneal structures that parallel the psoas muscle. The left kidney is usually slightly higher than the right and is slightly more medially located. The vertical axis of the kidneys, when compared with the midline, is about 20°. There is often considerable mobility of the kidneys as a result of respiration and body position. Several centimeters of excursion have been demonstrated on deep inspiration or in the upright position. During late embryological development, each kidney occupies the flank region, capped by the liver on the right side and the spleen on the left. Abnor­ malities of the liver and spleen can affect the position of the kidneys. Also, retroperitoneal masses may displace the kidney. A palpable abdominal mass which radiographically may appear to be an intraperitoneal structure can be accurately localized as a retroperitoneal tumor by observing displacement of the kidney, particularly if the kidney is pushed caudally and medially, superi­ orly and laterally, or medially. Anomalies of the kidneys include abnormal position, abnormal number, changes in shape, and alterations in the inner structure that may affect the renal parenchyma, the pelvocalyceal systems, or both.