Cochlear hair cells respond with phenomenal speed and sensitivity to sound vibrations that cause submicron deflections of their hair bundle. Outer hair cells are not only detectors, but also generate force to augment auditory sensitivity and frequency selectivity. Two mechanisms of force production have been proposed: contractions of the cell body or active motion of the hair bundle. Here, we describe recently identified proteins involved in the sensory and motor functions of auditory hair cells and present evidence for each force generator. Both motor mechanisms are probably needed to provide the high sensitivity and frequency discrimination of the mammalian cochlea.