The use of microwave and millimeter wave beamed energy for propulsion of vehicles in the atmosphere and in space has been under study for at least 35 years. The need for improved propulsion technology is clear: chemical rockets orbit only a few percent of the liftoff mass at a cost of over $3,000/lb. The key advantage of the beamed power approach is to place the heavy and expensive components on the ground or in space, not in the vehicle. Early efforts to use microwaves in propulsion beamed at high average powers to heat rocket engine fuel for inter-orbital transfers from low earth orbit to the moon and Mars. In the past two decades, microwave sources have been developed to extraordinary peak powers over a wide frequency range and are now operating at repetition rates in excess of 100 Hz, giving average powers of -40 kW.1 Development of these sources has preceded in several parameters: a general movement to higher power, development of high power sources at increasingly higher frequencies and higher repetition rates at all frequencies. Fig. 1 shows the present state-of-the-art of peak power as a function of frequency.