Waves moving through a medium in which the wave velocity is a function of time and space suffer frequency changes. If the velocity modulation is small and adiabatic, very elementary kinematic considerations define these frequency changes. For a uniformly moving velocity pattern the frequency change suffered by a portion of the wave is shown to depend only on the wave velocity at the input and output end of the modulating system, and not on the intermediate velocities experienced by the wave. Electro-optic modulation and deflection experiments are treated in this framework. The paper is intended to present a particularly simple viewpoint, and does not arrive at really new results. © 1963 The American Institute of Physics.