The nonlinearities in p-i-n photodiodes have been measured and numerically modeled. Harmonic distortion, response reduction, and sinusoidal output distortion measurements were made with two singlefrequency offset-phased-locked Nd: YAG lasers, which provided a source dynamic range greater than 130 dB, a 1 MHz to 50 GHz frequency range, and optical powers up to 10 mW. A semi-classical approach was used to solve the carrier transport in a one-dimensional p-i-n photodiode structure. This required the simultaneous solution of three coupled nonlinear differential equations: Poisson's equation and the hole and electron continuity equations. Space-charge electric fields, loading in the external circuit, and absorption in undepleted regions next to the intrinsic region all contributed to the nonlinear behavior described by these equations. Numerical simulations were performed to investigate and isolate the various nonlinear mechanisms. It was found that for intrinsic region electric fields below 50 kV/cm, the nonlinearities were influenced primarily by the space-charge electric-field-induced change in hole and electron velocities. Between 50 and 100kV/cm, the nonlinearities were found to be influenced primarily by changes in electron velocity for frequencies above 5 GHz and by p-region absorption below 1 GHz. Above 100 kV/cm, only p-region absorption could explain the observed nonlinear behavior, where only 8 to 14 nm of undepleted absorbing material next to the intrinsic region was necessary to model the observed second harmonic distortions of -60 dBc at 1 mA. Simulations were performed at high power densities to explain the observed response reductions and time distortions. A radially inward component of electron velocity was discovered, and under certain conditions, was estimated to have the same magnitude as the axial velocity. The model was extended to predict that maximum photodiode currents of 50 mA should be possible before a sharp increase in nonlinear output occurs. For capacitively-limited devices, the space-charge-induced nonlinearities were found to be independent of the intrinsic region length, while external circuit loading was determined to cause higher nonlinearities in shorter devices. Simulations indicate that second harmonic improvements of 40 to 60 dB may be possible if the photodiode can be fabricated without undepleted absorbing regions next to the intrinsic region.