Cochlear hair cells are mechanoreceptors of the auditory system and cannot spontaneously regenerate in adult mammals; thus hearing loss due to hair cell damage is permanent. In contrast, hair cells in nonmammalian vertebrates such as birds and in the zebrafish lateral line have the ability to regenerate after hair cell loss. Many regulatory factors, including signaling pathways, transcription factors, and epigenetic regulators, play roles in hair cell regeneration in various species. In this chapter, we review the history of hair cell regeneration research, the methods used in the study of hair cell regeneration, the properties and modulating factors of inner ear stem cells, and the re-formation of cochlear ribbon synapses and hearing function recovery.