The chapter Implantable hearing interfaces describes the fundamental operation of a commonly available biohybrid system, the cochlear implant, or bionic ear. This neuro-stimulating biomedical implant is very successful in restoring hearing function to people with profound hearing loss. The fundamental operation of the biological cochlea is described and parallels are drawn between key aspects of the biological system and the biohybrid implementation: dynamic range compression, translation of sound to neural activity, and tonotopic mapping. Critical considerations are discussed for simultaneously meeting biological, surgical, and engineering restrictions in successful biohybrid systems design. Finally, challenges in present and future cochlear implants are outlined and directions of current research given.