In developed countries 50% of childhood hearing impairment is attributable to genetic causes. In a limited number of cases, the hearing impairment is part of a syndrome, and several genes for syndromic deafness have been identified over the last 10 years. In the majority of cases, the hearing impairment occurs without additional clinical abnormalities (non-syndromic). Progressive hearing loss is very frequent in adults. By the age of 80 approximately 50% of the population is affected by age-related hearing loss, which is due partly to genetic factors. Before 1994, little was known about the genes responsible for non-syndromic hearing impairment, although epidemiological studies have suggested that more than 100 genes might be involved. Over the last 6 years, extremely rapid progress was realized in the field of the molecular genetics of hearing and deafness. More than 70 genes for non-syndromic hearing impairment have been localized to the human genome, and 18 of these have been identified.