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Abstract: As the gateway to human communication, the sense of hearing is of enormousAs the gateway to human communication, the sense of hearing is of enormousimportance in our lives. Hearing commences with the capture of sound energy by haircells, the ear's sensory receptors, which convert that energy into electrical signals thatthe brain can then interpret. Each hair cell is a cylindrical epithelial cell surmounted by ahair bundle, an erect cluster of 20-300 rigid, actin-filled rods termed stereocilia.Mechanical force deflects the hair bundle and thereby excites the hair cell and itsassociated nerve fibers.Uniquely among our sensory receptors, the hair cell is not a passive recipient ofstimuli, but instead uses an active process to enhance its inputs. This active processamplifies mechanical stimuli by as much as a thousandfold, greatly increasing oursensitivity to weak sounds. Amplification is accompanied by frequency tuning, whichyields a frequency resolution of less than 0.2 %, one thirtieth of the interval between pianokeys. The active process produces a compressive nonlinearity that renders the earsensitive to sounds over a millionfold range of amplitude or a trillionfold range of power.Finally, the active process can be so exuberant as to become unstable; as a result, in avery quiet environment most normal ears spontaneously emit sound!As a result of the cooperative gating of mechanically sensitive ion channels, a hairbundle is dynamically unstable: the relation between the bundle's displacement and theforce required to accomplish it possesses two stable fixed points separated by a regionof negative stiffness. This situation fosters amplification or oscillation when the hairbundle is pushed into its region of instability by molecular motors, specifically the myosinmolecules associated with adaptation of the transduction apparatus to sustained stimuli.Experiments on individual hair bundles indicate that the bundle's operation near thisinstability—a Hopf bifurcation—accounts for the four characteristics of the active process.