The principles of the travelling-wave valve as a phase-modulator and frequency-shifter are discussed. Measurements made on a typical valve operating at 4 000 Mc/s are recorded and show that about ± 2.5 radians linear phase shift is obtainable with a modulating signal of ± 50 volts injected in series with the beam voltage. Larger phase shifts may be obtained by applying the modulating voltage to a number of valves in tandem.Travelling-wave valves may also be used to shift the frequency of a carrier, e.g. from 4 000 to 4 035 Mc/s, by injecting a 35-Mc/s sinusoidal voltage in series with the beam voltage. When used in this way the gain and power output are only about 6 db below those obtained with the same valve used as an amplifier. The possible applications of travelling-wave valves as phase-modulators and frequency-shifters in the transmitters and repeaters of radio relay systems are discussed.