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...