The photoklystron is a photoelectric vacuum tube designed to convert broadband light energy directly into monochromatic high frequency AC power. It requires a DC reflecting voltage (which draws no power) and the first model works best with a DC accelerating voltage (which represents an additional input energy) but has been shown to operate with only light energy. With the accelerating voltage, frequencies from 4 to over 2 MHz. have been produced, and electrical efficiencies near 2% have been measured. Computer analysis of the electron trajectories revealed that unlike a reflex klystron the photoklystron works by extracting the kinetic energy of the electrons over a period of several cycles. For net energy output the accelerating and reflecting voltages are adjusted so that the electron trajectories have a period which is a multiple of the period of the RF output desired. Any electrons which are improperly phased are quickly eliminated by collision with the photocathode or repeller. Conditions for optimum interaction of the electrons with the RF field have been investigated both analytically and numerically using a Tektronix 452 computer. Although some aspects of the photoklystron are not fully understood at this time, it has been shown to be a fundamentally new and simple oscillator, offering a wide variety of applications.