Radio frequency vacuum microelectronics combines the advantages of electron transport in vacuum with gated electron emission structures derived from solid-state microfabrication. The advent of practical gated vacuum emitters of micron size will have a strong impact on rf source technology. Next-generation rf amplifiers incorporating microfabricated emitters will allow amplifier designs that minimize the need for high-voltage power supplies, complex modulating circuitry, and the heavy magnets common with linear beam tubes. One attractive application for such efficient compact amplifiers is as the vacuum power booster of a microwave power module, a device that combines a solid-state preamplifier with a low-gain vacuum amplifier for efficiency. Program goals are to create a 50 W 10 GHz amplifier with 10 dB gain and efficiency exceeding 50%. We will survey the requirements placed on field emitter arrays for performance in a microwave amplifier as determined by various analyses, describe the present status of the performance of ring cathodes designed for inductive output amplifiers, i.e., the narrow-band, cavity-based klystrode under development at Varian/CPI and the wide-band, helix-based twystrode under development at NRL, and identify the challenges that remain before this new rf source technology is realized. [S0734-211X(98)02702-4].