The kidney is one of the most common organs in which cysts develop. A wide variety of renal diseases may have cysts of the kidney associated with them, including hereditary, developmental, and acquired disorders. Renal cysts may arise from any part of the nephron or the collecting tubule and may be located in the cortex, medulla, or both. Classification is often difficult, because the cysts often resemble one another. Sonography has become increasingly useful in the diagnosis of pediatric renal cystic disease and in evaluation of fetal renal abnormalities in utero and can help narrow the differential diagnosis.