The spin catalysis of chemical reactions is a physical phenomenon of spin transformation of chemically reactive species induced by the exchange interaction of these species with external spin carriers. It manifests itself in the radical pair recombination, cis-trans isomerization of molecules with double bonds, recombination of spin-polarized and spin-aligned hydrogen atoms at cryogenic temperatures, etc. The theory and quantitative kinetics of spin catalysis in three- and four-spin systems are considered. Quantitative hierarchy of spin carriers with respect to their spin catalytic efficiency is discussed.