A sensory receptor of the internal ear, or hair cell, responds to sound or acceleration when this mechanical stimulus deflects the cell's mechanosensitive organelle, or hair bundle. The gating-spring model posits that mechanoelectrical transduction occurs as mechanical force is transmitted through an elastic element, or gating spring, to the molecular gate of each transduction channel; increased tension in the gating spring then promotes the channel's transition from a closed to an open state. Electrophysiological and micromechanical data from a variety of hair cells, both in vivo and in vitro, confirm that the stimulus dependence of channel open probability and bundle stiffness are quantitatively consistent with the model. The results accord still better, however, with an extended formulation including channel transitions among one open and two closed states. In addition to providing a derivation of this three-state model, this review delineates several experimentally testable predictions of gating-spring models.