An account is given of the work of L. I. Mandel'shtam and of his school in the field of interference at radiofrequencies between coherent radiations with commensurable frequencies, using direct measurements of the phase difference between the interfering oscillations. The various radiointerferometers used under the general direction of L. I. Mandel'shtam in studies of the phase structure and the velocity of radio waves over the earth's surface are described. Possible future developments in radiointerferometry and its applications in various scientific and practical areas are outlined.