Pkd1 encodes PC1, a transmembrane receptor-like protein, and Pkd2 encodes PC2, a calcium channel, which interact to form functional polycystin complexes that are widely expressed in many tissues and cell types. The study of autosomal dominant polycystic kidney disease (ADPKD), caused by inactivating mutations of PKD1 or PKD2 genes, has elucidated the functions of polycystins and their interdependence on primary cilia in renal epithelial cells. We have found that Pkd1 and Pkd2, as well as primary cilia, are present in osteoblasts and osteocytes. In addition, we have found that loss of polycystin-1 (Pkd1) function in mice results in abnormal bone development and osteopenia due to the impaired differentiation of osteoblasts. It is likely that the polycytin/primary cilia complex responds to a multitude of environmental clues affecting skeletal development and bone formation postnatally. Overall, polycystins in bone may define a new target for developing anabolic agents to treat osteoporotic disorders.