Describes the normal development of the ear; updates the classification and epidemiology of hearing loss; surveys the usage of audiometric tests and diagnostic medical examinations; illuminates the most important syndromes associated with hearing loss such as Usher, Pendred, and Waardenburg syndromes; discusses the maternal inheritance patterns of mitochondrial hearing loss; provides a roadmap to gene localization and isolation in nonsyndromic hearing loss, exploring connexins, tectorins, myosins, ion channels, and other disease genes; highlights otosclerosis, one of the most common types of hearing loss; considers mechanisms regulating hair cell differentiation and regeneration; and elucidates the future of genetic testing for hearing loss.