Novosibirsk: Sib. from . USSR Academy of Sciences, 1962. — 239 s. The proposed monograph was an attempt by Lev Ovsyannikov to revive Lee's theory in relation to specific systems of differential equations found in applied theories. The book discusses the following main problems of the theory of group properties of differential equations:1. Finding the widest continuous group allowed by a given system. 2. Group classification of systems of a given type . 3. Using a known group allowed by the system to build classes of private solutions of this system. Preliminary information from Lie group theory. Groups allowed by differential equations. Invariant-group solutions. Group classification of differential equations. Partial solutions of gas dynamics equations . Linear equation of the second order .