The general objectives are the understanding of the hearing process in man and animals under normal and abnormal conditions. The principal methods of investigation include (1) recording of electrophysiological activities of the ear during stimulation with sounds, including cochlear potentials and auditory nerve impulses, (2) determinations of auditory acuity by psychophysical and other procedures, including conditioned responses and electrodermal reactions, and (3) a correlation of electrophysiological and behavioral results with anatomical studies on the ear. For the most part a comparative approach is employed, with consideration of the various solutions to be obtained in animals at different levels in the vertebrate series. A unifying concept in this relation is the evolution of the ear.