Keynote paper at the May 30, 1979 joint scientific session of the Division of General Physics and Astronomy and the Division of Nuclear Physics of the USSR Academy of Sciences, which was devoted to the 100TH anniversary of the birth of Academician L. I. Mandel'shtam. The paper discusses the three main periods in Mandel'shtam's life and work: the Strasbourg period (1899--1914), the time from his education in Russia to his final move to Moscow (1914--1925), and the last, Moscow period (1925--1944), which was the most productive from the standpoint of both scientific and teaching activity. Mandel'shtam was (together with Academician A. F. Ioffe) the founder of Soviet physics, the creator of a physical school exceptional for breadth of its interests and the depth of its research, several of whose representatives went on to form widely known schools of their own, both theoretical (I. E. Tamm, A. A. Andronov, M. A. Leontovich) and experimental (G. S. Landsberg). His coverage of physics, the organic fusion of theoretical and experimental and mathematical and technical physics in Mandel'shtam's creativity, place him among the classics of science. The most important of Mandel'shtam's outstanding scientific and technical achievements are indicated, including the discovery of combination [Raman] and selective scattering of light, establishment of the role of fluctuations in scattering and prediction of the fine structure of Rayleigh scattering, his advancement of the theory of oscillations to the level of an independent scientific discipline and the creation of a nonlinear theory of oscillations, the invention of radiointerferometry, the discovery of the tunnel effect in quantum mechanics, etc. Also outlined are a number of general guiding concepts that Mandel'shtam developed, such as ``nonlinear physical thinking'', classification of phenomena on the basis of commonality (isomorphism) of the describing relationships, and application of ``oscillatory mutual aid'' between various fields of physics and engineering. These general ideas unified and directed all of his creative work and have by now thoroughly permeated modem physics.