Thirty million Americans have significant hearing problems that range in severity from modest difficulty with speech comprehension to profound deafness. A comparable fraction of the population, about 10%, is afflicted in other industrialized countries as well. The majority of the hearing-impaired suffer from sensorineural hearing loss. Despite the common name for this condition, "nerve deafness," sensorineural hearing loss usually results from damage to the sensory cells of the inner ear, rather than to the associated nerve. Each cochlea normally contains about 16,000 of these receptors, which are termed hair cells. Similar hair cells in the vestibular labyrinth mediate our sense of equilibrium, or responsiveness to linear and angular accelerations. Because they are not mitotically replaced when damaged, human hair cells are continually lost throughout life as a result of genetic deficits, infections, ototoxic drugs, acoustical trauma, and ageing. In the hope of laying the groundwork for an understanding of normal hearing, as well as its deterioration and restoration, the members of our group are investigating the structure and operation of hair cells. We employ a variety of technical approaches, including those of biophysics, electrophysiology, developmental biology, biochemistry, genetics, molecular biology, immunology, and ultrastructural analysis. In addition, we attempt to assimilate our findings into mathematical models of hair-cell function.