Monte Carlo simulation has been shown to be an effective approach to the study of ultrafast carrier relaxation in semiconductor bulk materials and in microstructures. We will review the use of this methodology to study electron-electron and electron-hole interactions, non-equilibrium and confined phonons, and inter-subband relaxation in quantum wells. We will also discuss the presence of the collision-duration on the short-time scale, and will review the work of some other workers in the field. Finally, we discuss some of die limitations of die Monte Carlo technique.