Structural defects associated with deafness may be divided into two classes. The first class is associated with structures of the external and middle ear with the result that conduction of sound is impeded. Deformities of the external auditory canal and the ossicles may be hereditary and present at birth. Middle-ear infections, otosclerosis, etc., acquired later may alter normal ossicular chain function. In the second class, the pathology is restricted to the internal ear. If the defect is congenital, it may have been inherited, or occurred “in utero” such as with rubella infections. After birth, an individual may be exposed to any number of situations which cause destruction of the hair cells and the cochlear nerve fibers. If the loss is extensive enough, a defective receptor organ results, with partial or complete deafness of the individual. It is not clear at present precisely what percentage of hair cells and nerve fibers are necessary for adequate social communication, but there is no replacement of degenerated receptor cells or sensory nerve fibers.