  Since the first vacuum tube (X-ray tube) was invented by Wilhelm R\"ontgen in
Germany, after more than one hundred years of development, the average power
density of the vacuum tube microwave source has reached the order of 108
[MW][GHz]2. The maximum power density record was created by the Free Electron
Lasers. In the high-power microwave field, the vacuum devices are still the
mainstream microwave sources for applications such as scientific instruments,
communications, radars, magnetic confinement fusion heating, microwave weapons,
etc. The principles of microwave generation by vacuum tube microwave sources
include Cherenkov or Smith-Purcell radiation, transition radiation, and
Bremsstrahlung. In this paper, the vacuum tube microwave sources were reviewed
in order according to the three radiation principles. Among them, the Vircators
can produce 22 GW output power in P-band (0.23-1GHz). Vacuum tubes that can
achieve continuous-wave operation include Traveling Wave Tubes, Klystrons, Free
Electron Lasers, and Magnetrons, with output power up to 1MW. Vacuum tubes that
can generate frequencies of the order of 100 GHz and above include Klystrons,
Traveling Wave Tubes, Gyrotrons, Backward Wave Oscillators, Magnetrons, Surface
Wave Oscillators, Free Electron Lasers, Orotrons, etc. Gyrotrons are very
attractive in the millimeter wave and THz fields. The Gyrotrons can output
power at the MW level with 3000s pulse width at millimeter wavelengths.
