Since David Kemp first proposed the active model in 1979, the role of the outer hair cell (OHC) in basilar membrane (BM) mechanics has been hotly debated. In the ``active'' model view, the OHC modulates the motion of the BM traveling wave on a cycle-by-cycle basis, leading to a negative BM resistance and a traveling-wave power gain. Nonlinearity is introduced by assuming that the negative BM resistance depends on the signal level. In the ``passive'' model view, the OHC controls the stiffness of the basilar membrane, leading to a level-dependent (nonlinear) relative impedance between the BM and tectorial membrane and a nonlinear basilar membrane and transduction response. Both models seem to be able to achieve the important nonlinear variations in responses seen in the BM, OHC, IHC, and neural response, but with differing assumptions and degrees of physical reality. It is now clear that the OHC nonlinearly compresses both the dynamic range of basilar membrane motion (Rhode, 1971; Ruggero, 1990) and the neural response (Yates, 1989), extending the otherwise limited dynamic range of the IHC response. This role of the OHC may be quantified using psychoacoustic masking patterns, two-tone suppression, loudness growth and recruitment, and OAEs as objective measures.