Renal cysts are relatively common. They may be single, multiple, or innumerable, cortical or medullary. Most renal cysts are spherical, thin-walled, epithelial-lined structures that contain fluid with features of a plasma ultrafiltrate. A clinically useful classification must include characteristics such as age of onset, family history and association with extrarenal lesions, as well as morphologic parameters. Renal dysplasia is disturbed renal histogenesis, either syndromic or non-syndromic, which usually gives rise to morphologically characteristic renal cysts. Two important and distinctive familial forms of renal polycystic disease occur. Autosomal dominant polycystic disease characteristically becomes manifest in adulthood but is becoming increasingly diagnosed in young individuals. Autosomal recessive infantile polycystic disease has complex relationships with "congenital hepatic fibrosis". Other cystic renal lesions are less common in young patients, but their occurrence in association with other abnormalities provokes speculation regarding pathogenesis.