The influence of an external magnetic field on the chemical and photochemical processes in molecular solids, semiconductors, photosynthetic systems, and liquid solutions, the magnetic (or nuclear-spin) isotope effect, the chemically induced magnetic polarization of electrons and nuclei, the radiofrequency chemical maser, the high-frequency magnetic-resonance modulation of the rates of physical and chemical processes involving paramagnetic particles, and magnetic effects in the molecular physics of gases--all these effects are a consequence of spin selection rules and spin evolution. In this review, we analyze the origin of magnetic spin effects, estimate their quantitative scale, and discuss the time scale of spin dynamics.