Several models of cochlear mechanics have been developed. Wave motion in the cochlea is described by most of these models in a manner analogous to traveling waves in an electrical transmission line. To model the frequency response of the basilar membrane, the electrical properties of the transmission line must vary along the length of the line. Linear models match the frequency response characteristics of the basilar membrane quite well;Psychophysical and neurophysiological observations of combination tones indicate the presence of a nonlinear mechanism within the cochlea, but the existence of a mechanical nonlinearity has not yet been proved. The purpose of this investigation is to develop a theoretical justification for nonlinearities in cochlear mechanics. Newtonian theories of fluid and solid mechanics are applied to a simplified physical model of the cochlea. Each nonlinear term in the equations of motion is considered, and its effect on the wave motion in the cochlea is discussed. A cubic nonlinearity results from the nonlinear nature of the compliance of the cochlear partition. A quadratic nonlinearity may also exist if the effects of fluid convection are significant. These results are consistent with existing nonlinear models which exhibit combination tone behavior similar to that of the cochlea. It is concluded that a mechanical nonlinearity is predicted by a simply physical model of the cochlea. Odd-order distortion products, such as the 2f(,1)-f(,2) combination tone, are consistent with the expected effects of nonlinear compliance of an elastic cochlear partition.