We have shown here several examples of how hearing and vestibular impaired mouse mutants are generated and the insight that they provide in the study of auditory and vestibular function. These types of genetic studies may also lead to the identification of disease-susceptibility genes, perhaps the most critical element in presbyacusis (age-related hearing loss). Some individuals may be more prone to hearing loss with increasing age or upon exposure to severe noise, and susceptibility genes may be involved. Different inbred mice show a variety of age-related and noise-induced hearing loss that varies between normal hearing and severe deafness throughout their life span /27/. Genetic diversity between inbred mouse strains has been shown to be a powerful tool for the discovery of modifier genes. Already two studies have found regions in which modifier genes for deafness may reside /28-29/. Future studies will hopefully lead to the identification of genes that modify hearing loss and will help us understand the variability that exists in human hearing, a crucial component in developing successful treatment strategies. The first human non-syndromic deafness-causing gene was identified in 1995, and since then, additional genes have been discovered. Much of the credit for this boom is due to deaf and vestibular mouse mutants. Their study has led to great insight regarding the development and function of the mammalian inner ear, and correlations with human deafness can now be made since mutations in the same genes have been found in these two mammals. As deafness is the most common form of sensory impairment and affects individuals of all ages, elucidating the function of the auditory and vestibular systems through genetic approaches is essential in improving and designing effective treatments for hearing loss.