The inner ear is a highly specialized organ and the mechanisms of its function are complex and have not yet fully been elucidated. For example, there are questions such as how the stereocilia characteristics of hair cells are regularly arranged, how the reactions of stereocilia and ion channels of hair cells to sound are controlled, and how the ion environment is maintained in the internal ear. The mechanisms of inner ear function are being elucidated by analysis of human hereditary hearing disorders and genetic and molecular biological techniques using mouse hearing disorder models. Understanding of the mechanism of inner ear function provides important information for treatment of the inner ear. This review outlines several findings obtained from humans with hereditary hearing disorders and mouse hearing disorder models.