The simple Doppler-shift formula fD = v/? shows that frequency changes (fD) useful for actual practice have to be based on either rapid changes of the distance between transmitter and receiver (v) or the application of very short waves (?), and this with high frequency stability. Both conditions became feasible only during the last twenty years because of rapid technological progress. Some applications of the Doppler effect are described in connection with a number of modern radio-location systems.