Each pair of meshing gears in a transmission gives rise to a source of vibratory excitation that can result in the radiation of sound. Each such source is most conveniently characterized as a displacement form of excitation generally referred to as the static transmission error of the gear pair. Contributions to the frequency spectrum of the static transmission error of spur and helical gears arising from tooth and gear body elastic deformations and from deviations of tooth surfaces from perfect involute surfaces are considered. Tooth surface deviations are decomposed into contributions giving rise to tooth meshing harmonic excitations and rotational harmonic or sideband excitations. Various types of gear tooth errors are defined and the contributions of these errors to different parts of the frequency spectrum are described. The attenuating effect on the static transmission error spectrum arising from the smoothing action of multiple tooth contact is explained.