In addition to responding to mechanical stimuli, the hair cell's transduction apparatus mediates active hair-bundle motility, one mechanism underlying the active process that increases responsiveness to sound, sharpens frequency selectivity, compresses the dynamic range of hearing, and even causes spontaneous otoacoustic emissions. In non-mammalian tetrapods-and perhaps in mammals as well-mechanical amplification is accomplished by active hair-bundle motility, which results from the interaction of negative hair-bundle stiffness with the myosin 1c motors that underlie adaptation. The operation of the active process near a Hopf bifurcation explains many of the characteristics of hearing. In particular, the dependence of response amplitude on stimulus force is expected to follow a power law with an exponent of one-third, as is measured experimentally. Operation near a Hopf bifurcation additionally produces distortion products with the level dependence observed for human hearing. Finally, a critical oscillator can become unstable, providing a natural explanation for spontaneous otoacoustic emissions.