Cystic kidneys or renal cystic disease is a morphologic description for an etiological heterogeneous group of disorders ranging from solitary cysts to several forms of multicystic and polycystic kidneys. The combination of the examination of the kidneys and liver, clinical data, family history and the presence of associated anomalies is mandatory to obtain a final diagnosis. The use of prenatal ultrasound to monitor pregnancies at risk for autosomal recessive polycystic kidney disease (ARPKD) is limited because a recurrence can be diagnosed early in pregnancy but may not be excluded. For pregnancies at risk for autosomal dominant polycystic kidney disease (ADPKD), a reliable prenatal diagnosis can only be provided by DNA studies after chorionic villus sampling. Cystic kidneys may present as part of different syndromes. An overview is given of the complex differential diagnosis. Dysplastic (multicystic) kidneys often occur unilaterally. In contrast with polycystic kidneys, diseased liver changes are not present in cystic dysplasia and prenatal ultrasound diagnosis is usually possible.