Spin exchange between paramagnetic particles is one of the few elementary bimolecular processes in solutions that is readily accessible to detailed experimental studies and to strict theoretical description. Due to this fact spin exchange is now widely used in solving various problems of chemistry, physics, and molecular biology. Spin exchange allows one to study in detail elementary acts of collisions of molecules. By use of spin exchange one can obtain quantitative information on collision rates of molecules in solutions, steric hinderance in collisions, electrostatic interactions in collisions of charged particles, and overlap of electron orbitals in collisions. Par­ ticularly valuable is the possibility of using spin exchange to study col­ lisions in such complex systems as polymer solutions, multicomponent mix­ tures, heterogeneous mixtures, and especially biological systems. This monograph contains a comprehensive description of spin exchange in solutions. It consists of four chapters. The introductory chapter contains a brief summary of the physical principles of spin exchange and outlines the range of its main applications. In this chapter a historic background of research in this field is also given. The second chapter contains a comprehensive presentation of the theory of spin exchange including that of its spectroscopic manifestations. The third chapter is devoted to a detailed description of the technique of experimental studies of spin exchange. The last - the fourth - chapter summarizes the results of experimental studies of spin exchange and considers various applications of spin exchange in che­ mistry, molecular physics, and biology.