The paper reviews the various types of electron vacuum tubes employed for amplification and generation at microwave frequencies. Emphasis is placed on principles of operation and on tubes recently developed to give high power output, oscillations at the highest frequencies and low noise factors. The treatment is restricted to conventional tubes in which the output energy is derived from the d.c. input. The subject is interpreted in terms of published work, the text being closely associated with a bibliography which is complete up to the Microwave Valve Convention of May, 1958.After a general introduction, the first part discusses grid-controlled tubes. It is then shown how the interaction of space-charge waves with resonant cavities and slow-wave circuits results in the various forms of drift-space and growing-wave tubes. The second part deals with crossed-field interaction in planar and circular geometry and includes an examination of the magnetron. An account is given of novel methods of generation of submillimetre waves and the usual sources of electrons are described. The third part analyses noise phenomena in oscillators and amplifiers.