The chapter presents the method of tooth contact analysis of cylindrical helical gears to determine the transmission error excitation. The transmission error was calculated based on the geometry of mating teeth surfaces with topography errors. These errors can be introduced to analyses from measurements of actual gears. The obtained geometrical transmission error is then used to calculate its frequency spectrum (Fourier transformation). Next, the transmission error spectrum was compared with the spectrum of the actual gear case acceleration signal in the power circulating test rig to evaluate its components and the influence on the gearbox’s operation.