A vacuum tube that is suitable for use as an amplifier or oscillator at microwave frequencies. These tubes usually employ velocity modulation of the electron beam rather than density modulation as in the valves used in audiofrequency valve amplifiers or oscillators.

Microwave tubes may be classified into two main types: linear-beam tubes, in which the electron beam travels in an essentially linear direction, and crossed-field tubes, in which the electron beam follows a curved path under the influence of orthogonal electric and magnetic fields.

The klystron and most forms of travelling-wave tubes are linear-beam tubes; the magnetron is a crossed-field tube.