Publisher Summary This chapter discusses the ultrastructure of the organ of Corti with special emphasis on mechanoreceptors and neural elements. The organ of Corti is a papillary structure resting on the basilar membrane, which is composed of sensorineural and supportive elements. It is specially designed to convert mechanical vibrations into electrical events which are transmitted to the central nervous system as coded messages. There are two types of mechanoreceptors, outer and inner hair cells. The inner hair cells are supported by inner pillar cells, inner phalangeal cells, and border cells. The outer hair cells are supported by outer and inner pillar cells and Deiters' cells. More peripheral to these structures are inner sulcus cells, Hensen's cells, Claudius cells, and Boettcher cells. The bases of all these cells rest on the basilar membrane; at the top the stereocilia of the hair cells attach to the tectorial membrane. The general organization of the organ of Corti suggests that it is most uniquely designed to transmit small mechanical movement effectively into the hair cells, and at the same time to withstand strong mechanical vibrations.