On the basis of the current experimental data, a survey is given of our knowledge of the physical nature and mechanism of direct nuclear reactions--processes in which most of the energy and momentum of the incident particle is transferred to a single nucleon or to a relatively small group of nucleons. It is shown why it is particularly advantageous to make use of high-energy particles to investigate the nature of direct reactions. The interrelationship of nuclear physics and elementary particle physics is discussed. An outline is given of the theoretical fundamentals of the study of direct reactions. Consideration is given mainly to the determination of the mechanism of direct reactions involving three or more particles in the final state. The pole and triangle Feynman diagrams, as well as an approximate method of allowing for the contribution of other diagrams, are discussed in detail. The following types of reactions are considered: quasielastic processes, double charge exchange of pions, elastic $pd$ scattering, the K-$d$ interaction, and the nuclear capture of pions and kaons. The experimental data are analyzed and compared with the theoretical results. Desirable directions of further experimental investigations are indicated.