A theory of large-signal Impatt oscillator operation, including the effects of both constant and modulated optical carrier generation, is developed. This theory is related to easily measurable parameters of the particular device and predicts useful oscillator locking ranges for modest modulated optical-power levels. Computer simulations of practical device structures, under optical control, supplement the analytical work. The paper concludes with some recent results from an experiment in which optical subharmonic locking of an X-band Impatt oscillator has been achieved.