Mammalian cochleae are “snail-like” hearing organs in the inner ear, containing the sensory epithelium, called the organ of Corti, where mechanosensory hair cells are arranged in rows along the entire length of the cochlear coil. Hair cell differentiation proceeds from the basal to the apical region of the cochlea, but the mechanism and significance of this remain to be elucidated. Our colleagues and I investigated the role of Hedgehog (Hh) signaling in cochlear development sing transgenic mice, and found that Hh signaling delays hair cell differentiation in the apical region, which forms the basal-toapical wave of development. Downregulation of Hh signaling caused hair cell disarrangement and hearing impairment suggesting that the basal-to-apical wave is required for hearing ability.