For quite some time the field of polycystic kidney disease has led a life at the fringe of kidney research, but with the cloning of the PKD1 and many other genes this situation has dramatically changed. Polycystic kidney disease often is a syndromic disease affecting a variety of organs in addition to the kidney. Most of the proteins involved in polycystic kidney disease have been localized to the primary cilium, an extension at the apical membrane of renal tubular epithelial cells, which may serve chemo- and mechanosensory functions. It is speculated that primary cilia and their associated proteins play a role in determining the proper tubular geometry.