Nearly two-thirds of people aged over 70 have some degree of hearing impairment (see DEAFNESS). Hearing aids are no substitute for definitive treatment of the underlying cause of poor hearing, so examination by an ear, nose and throat surgeon and/or an audiologist is sensible before a hearing aid is purchased or supplied. The choice of an aid depends on the age, manipulative skills, and degree of hearing impairment of the patient, and on the underlying cause of the deafness. The choice of hearing aid for a deaf child is particularly important, as impaired hearing can hinder speech development.

Electronic aids consist, essentially, of a microphone, an amplifier, and an earphone. In postaural aids the microphone and amplifier are contained in a small box worn behind the ear or attached to spectacles. The earphone is on a specially moulded earpiece. Some patients find it difficult to manipulate the controls of an aid worn behind the ear, and they may be better off with a device worn on the body. Some hearing aids are worn entirely within the ear and are very discreet. The most sophisticated aids sit entirely within the middle-ear canal so are virtually invisible. They may be tuned to amplify only the frequencies the wearer cannot hear.

Many have a volume control, and a special setting for use with telephone and in rooms fitted with an inductive coupler that screens out background noise.