A process that introduces a radiofrequency (r.f.) component into an electron stream and thereby modulates the velocities of the electrons in the beam. Individual electrons will be either accelerated or retarded by the radiofrequency signal depending on the relative phase of the r.f. component at the point of interaction with the electrons. The velocity modulation therefore causes bunching of the electron beam as it travels down the electron tube since the faster electrons catch up with preceding slower ones (and conversely).

The amount of bunching of an electron stream varies with the distance travelled from the point of first interaction with the r.f. field. Ideal bunching is the production of small sharply defined bunches of electrons with no electrons in the regions between bunches. In practice however this is not achieved. Optimum bunching occurs at a particular distance down the tube for any given tube and is the condition when the minimum size of bunches containing the maximum possible numbers of electrons is achieved. Underbunching is the condition of less than optimum bunching. If the electron beam is allowed to travel beyond the point of optimum bunching faster electrons begin to leave the slower ones behind. This condition is termed overbunching. If the electron-beam direction is reversed by a reflector during the transit interval the resulting bunching is known as reflex bunching.

Velocity-modulated tubes are used as microwave oscillators and amplifiers. They include the klystron and the travelling-wave tube. The r.f. field can be made to interact with the electron stream in one sharply defined region, such as a cavity resonator; the region is known as the buncher. The electron beam is then allowed to travel through a field-free drift space. This method is employed in the klystron. An alternative method is used in travelling-wave tubes: the r.f. field is propagated along the length of the electron tube and interaction between the electron beam and the field is a continuous two-way process.