The mammalian inner ear is vulnerable to genetic disorders and aging, as well as to injuries caused by overstimulation, ototoxic drugs, and viral infections. Due to the poor regeneration of the sensory epithelium and the spiral ganglion neurons in the adult mammalian inner ear, cell replacement therapy strategies have been proposed to compensate for degeneration and loss of sensory and neuronal cells. Transplantation of stem cells and embryonic neurons into the inner ear has revealed that exogenous cells can survive, migrate, differentiate, and extend neuritic projections in the auditory system of adult mammals. These results suggest that cell replacement therapy could provide an effective future treatment alternative for hearing loss and other inner ear disorders.