Definition Hearing is the usual channel for acquisition of this most important of human attributes, language. Language enables humans to communicate at a distance and across time and has played a decisive role in the development of society and its many cultures. Language is the major channel through which children learn about what is not immediately evident, and it plays a central role in thinking and cognition. Because speech is the chief vehicle for communication in all families (except those in which the parents are deaf), deafness is a profound handicap whose effects greatly transcend the inability to speak. Responsibility for detecting hearing loss in infancy rests on the primary physician, inasmuch as early diagnosis and appropriate habilitation will prevent the most serious consequences of infantile hearing loss: growing up without language. Two primary types of hearing loss are attributable to disease of the ear: conductive hearing loss, a deficiency in the transduction of energy in the form of sound waves in air to hydraulic waves in the inner ear; and sensorineural hearing loss, inadequate transduction of these waves to neural activity. Other disorders of hearing include cortical hearing impairment and perceptual disorders. Although much is said about perceptual disorders by educators, such disorders probably play a minor role in the genesis of learning disabilities and will not be addressed in this review, which focuses on severe-to-profound hearing losses.