Electron scattering by various kinds of impurities in semiconductors was studied using millimeter wave and infrared cyclotron resonance, and it was found possible to use linewidth measurement to derive the inverse relaxation time of electrons due to impurities and hence the scattering coefficient as well as the cross section. The study ranged from well-identified impurity species in elemental semiconductors to more obscure ones in compound semiconductors, with both neutral and ionized impurity scattering being treated. The treatment was also extended briefly to dislocations and thermal defects. The results are tabulated for impurities in Ge and Si only, since they are basically extensible, mutatis mutandis, to impurity scattering in compound semiconductors.