Within the inner ear is the tiny, snail-shaped organ critical for hearing, the cochlea. The cochlea contains thousands of neural receptor cells, called hair cells for their appearance. Sound waves travel from the outer and middle ear to the inner ear, where they are changed by the hair cells into electrical signals conveyed by the auditory nerve to the brain. The brain interprets the signals as meaningful sound. Without hair cells, the process of hearing does not occur.

Damage to these delicate cells results in permanent hearing damage, since the body does not spontaneously replace them. Hair cell damage can be caused by a wide variety of agents, including loud or sustained noise, high fevers, certain viruses, some drugs and medicines, and aging (the exact mechanisms of age-related hearing loss remain poorly understood). One of the many kinds of hearing impairments, hearing loss due to hair cell damage or damage to the auditory nerve is called "sensori-neural" hearing loss.

80% of all cases of hearing loss are sensori-neural. In the United States, more than 22 million people have enough sensori-neural hearing loss to affect their ability to communicate. This form of hearing loss is particularly devastating because it affects the ability to discriminate between sounds and understand complex combinations of sounds, including speech.

Hearing aids provide only limited help, as they are amplification devices which can only stimulate remaining hair cells, not repair the cells that have died or been damaged. Hair cells are frequency-specific, so hearing aids only help a person perceive sounds in the frequencies for which hair cells remain. Thus, persons with sensori-neural hearing loss may experience greater difficulty in communication and other vital listening tasks, yet receive less help from assistive technology, than persons with other kinds of hearing loss.

Until recently sensori-neural hearing loss was considered irreversible, as there was no known way to replace or repair hair cells. However, it has been known that certain lower animals (fishes or amphibians, and some birds) can regenerate hair cells throughout their lifespans, and researchers have been able to stimulate hair cell regrowth in immature birds, such as chicks. In an important breakthrough reported by several labs in early 1993, mammalian ears were discovered to retain the "parent" cells that give rise to hair cells in fetuses. If scientists can learn how to trigger these cells to begin producing hair cells after birth, it might be possible to develop an effective therapy for sensori-neural hearing loss.