Cilia are classified according to their microtubule components as 9+2 (motile) and 9+0 (primary) cilia. Disruption of 9+2 cilia, which move mucus across respiratory epithelia, leads to rhinitis, sinusitis and bronchiectasis. Approximately half of the patients with primary ciliary dyskinesia (PCD) have situs inversus, providing a link between left-right asymmetry and cilia. 9+0 cilia at the embryonic node are also motile and involved in establishing left-right asymmetry. Most 9+0 cilia, however, act as antennae, sensing the external environment. Defective 9+0 cilia of principal cells of the nephron cause cystic diseases of the kidney. In the rods and cones of the retina, photoreceptor discs and visual pigments are synthesized in the inner segment and transported to the distal outer segment through a narrow 9+0 connecting cilium; defects in this process lead to retinitis pigmentosa. Although the function of primary cilia in some organs is being elucidated, in many other organs they have not been studied at all. It is probable that many more cilia-related disorders remain to be discovered.