The book presents an extended version of the lecture course on the theory of nuclear reactions that has been given by the author for some years in Kiev State University. An account is given of the nonrelativistic nuclear reaction theory. The R -- matrix description of nuclear reactions is considered and the dispersion method is formulated. Mechanisms of nuclear reactions and their relationship are studied in detail. Attention is paid to nuclear reactions involving the compound nuclear formation and to direct nuclear processes. The optical model, the diffraction approach and high -- energy diffraction nuclear processes involving composite particles are discussed. It also deals with some problems treated only in special journal papers.