Hair cells of the inner ear act as mechanosensors, converting mechanical stimuli to electrical signals transmitted to the brain. Hair cells of the auditory organs respond to sound stimuli for hearing perception; those in the vestibular organs respond to gravity and head movements for perception of balance. Hair cells are called such because they have actin-rich protrusions, stereocilia, at their apical end. Displacement of stereocilia opens ion channels resulting in depolarization and release of transmitter from synapses at the basal end of the cell to terminals of innervating afferent nerves. Damage and loss of hair cells are leading causes of hearing and balance disorders, affecting over 40 million people in the US. Hair cells are susceptible to environmental insults, including noise, chemical exposure and accumulated damage during aging. Hair cell loss in humans is irreversible. Zebrafish lateral line hair cells share many properties with those of the inner ear, and are sensitive to the same types of damaging agents as human cells. However they are able to regenerate hair cells throughout life. The location of the lateral line on the surface of the body allows access for visualization and manipulation. We use the zebrafish system to understand why hair cells die, how they can be protected from damage, and how they can regenerate.