The book was written by famous Soviet scientists who gave fundamental work in the new field of the theory of nonlinear oscillations, which is widely used in modern technology (automatic control, radio engineering, etc.). The book systematically presents extensive material on the theory of nonlinear oscillations of autonomous systems with one degree of freedom, covering a large number of oscillatory systems found in engineering practice. The first edition of the book was published in 1937; this edition repeats it unchanged, only with the correction of typos and typographic errors. The book is designed for scientific and engineering workers who encounter various oscillatory processes in their work.