Monte Carlo simulation has been widely used to study charge-carrier transport in electric fields in GaAs, InP, Si and Ge at high frequencies. New physical ideas for solution of the problems important for solid-state devices for microwave and far-infrared applications are presented and discussed: overshoot of drift velocity, avalanche plasma instabilities, plasma-wave excitation by drifted electrons in GaAs, inversion of population of light and heavy holes in p-Ge, negative differential conductivity due to intervalley and optical-phonon scattering in GaAs and heavy-hole negative effective mass. They have opened new possibilities to develop semiconductor amplifiers and oscillators at frequencies 10(11)-10(13) s-1. Some of the predictions are confirmed experimentally.