The book is the second edition of the classic work of academician N. N. Semenova "Chain reactions" (1934 g. ), which developed a general theory of chain and branch-chain chemical reactions. Based on the analysis of experimental data on the mechanism of chemical reactions (mainly in the gas phase), a number of general laws of chemical kinetics are considered and their theoretical justification is given from the point of view of the chain theory. and also shows the application of this theory to explain the experimentally observed patterns of certain chemical reactions. Annexes to the second edition include works by H. N. Semenov subsequent years, which reflect the ways and prospects of the chain theory, its connection with modernity At the end of the book there is a comment from the students of N. N. Semenova . The book is designed for researchers - chemists, physicists, as well as university professors, graduate students and senior students .