Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutation of one of two genes: PKD1 (16p13.3) or PKD2 (4q13–23). PKD1 accounts for ∼85% of pedigrees and is associated with significantly more severe cystic disease. The ADPKD genes encode proteins, polycystin-1 and polycystin-2, which are very different in size and structure, but which have a region of homology and may interact as part of the same complex. Polycystin-1 is a large, integral membrane protein (∼460 kDa) predicted to be involved in cell-cell and/or cell-matrix interactions. Polycystin-2 (∼110 kDa) is related to polycystin-1 and voltage-activated and transient receptor potential channel subunits, suggesting that the polycystins may also be associated with ion transport. A polycystin complex could regulate cellular events (that are abnormal in ADPKD) in response to specific extracellular cues, mediated by controlling cellular Ca²⁺ levels and/or other signalling pathways. Recently, two further polycystin-like molecules have been identified, indicating roles for this novel protein family beyond the kidney. A wide range of different mutations to the PKD1 or PKD2 gene have been detected, most predicted to truncate and inactivate the proteins. A somatic second hit may be required for focal cyst development, although there is widespread immunohistochemical evidence of polycystin expression in cystic epithelia. Disruption of the mouse Pkd1 gene leads to death in the perinatal period with massive cystic expansion in homozygotes and age-related cyst development in heterozygotes. Normal renal development in Pkd1^del34/del34 mice up to embryonic day ∼15.5 suggests a role for polycystin-1 in developing and maintaining the tubular architecture, consistent with the localization of the protein, ratherthan nephron induction. Renal cystic disease in homo- and heterozygotes of a Pkd2 mouse model with a disrupted exon 1 inserted in tandem with the normal exon (and prone to somatic recombination, which inactivates the gene) supports a role for somatic events in cystogenesis.