High-power klystrons, coupled-cavity traveling-wave tubes (TWT's), and hybrid tubes, all of which utilize the microwave cavity as the basic circuit element, are described. These amplifiers are used in communications, radars, electronic countermeasures, and other applications at power levels from a few hundred watts to megawatts, at frequencies from ultrahigh frequency on up, and are particularly suited for high average powers. High gain, 30 to 60 dB, is normally achieved, and bandwidth usually lies in the 1-30-percent range. Elementary theory of operation is described, together with design considerations and systems interface information. Typical tube designs and data are presented. Recent developments are discussed, including high-efficiency techniques (to 75 percent), improvements in bandwidth, periodic focusing, and beam control electrodes. Most of the basic design techniques are well developed, and emphasis is being shifted to improvements in the detailed performance characteristics such as gain and phase resolution ripples, noise, and the sensitivities to operating voltages and currents.