The performance of modern communications systems that use multicarrier modulation techniques such as, for example, OFDM(Orthogonal frequency division multiplexing), is highly sensitive to nonlinear distortions generated mainly by high power amplifiers (HPAs). In order to fully characterize these distortions, the memory effects of these devices must be considered. These effects become more important as the signal bandwidth increases. On the other hand, the increase in symbol duration makes multicarriers systems more sensitive to time variations as those in mobile channels. In particular, the Doppler effect tends to impair the orthogonality between sub-carriers, resulting in inter-carrier interference (ICI). Therefore, the study of the effects of nonlinearities and time-varying channels on OFDM systems performance becomes important, mainly in mobile communications systems. The studies currently available in the literature are limited to evaluating the effects of time invariant non-linearities or the effects of linear time varying channels. In this work, mathematical expressions are developed to evaluate the joint effect of nonlinearities and time-varying channels on OFDM systems. The investigation of these effects is performed at two levels: the effect on the spectrum of OFDM signal and effect on the system s performance. In deriving the analytical expressions related to both levels of investigation the time-varying non-linear channel was modeled by a Volterra Series. Numerical results, obtained for specific situations, illustrate the resulting impact on the OFDM spectrum (spreading) and on the system s bit error rate performance.