We humans hear the way we do because of at least three major forces. The first is phylogeny, the evolutionary changes in the auditory system since its beginnings. Another is embryology, the development of the system in each individual. Finally, there is the biologically determined mechanism we are born with and our interaction with the environment in early postnatal life. This series of articles reviews each aspect so that we may have a fuller appreciation of how it is we come to hear the way we do. Part I described the phylogeny of the auditory system. Part II traces the development in prenatal life of the internal, middle, and external ear and the nervous system by days, weeks, and months. Wherever possible, parallels are drawn between phylogenetic and embryologic development. Part III considers the postnatal aspects of auditory development.