Approximately 1 in 1000 children is born with a functionally significant, permanent hearing loss, and about 1 in 500 will have a hearing loss by the age of 9. Audiologic testing demonstrates that this hearing loss is attributable to dysfunction of the inner ear or the auditory nerve. Approximately one-half of these cases are caused by environmental factors such as infections, and the other half are genetic in origin. One-third of genetic cases are inherited as part of a syndrome that affects other organ systems. The other two-thirds exhibit hearing loss as the only clinical feature and are termed nonsyndromic. Early observations of relatively high frequencies of normal-hearing offspring born to two parents with nonsyndromic deafness indicated that there were many different genes in which mutations could cause hearing loss. Although hearing loss is increasingly prevalent among older adults, affecting as many as one in two individuals by the age of 80, the genetic basis of this age-related hearing loss, presbycusis, is not well understood. There is a genetic component, but the genetic basis appears to be complex and may involve more than one gene in any individual with presbycusis.