AbstractThe cationic polymerization of alkenes and heterocyclics is compared in a critical way. After a brief introduction to the polymerizability of various monomers and efficiency of initiators, various active sites involved in propagation are discussed. They include covalent species, onium ions and carbenium ions of different degrees of association. Proportions and reactivities of these active sites are discussed for different monomers. Structure‐reactivity correlations for both systems are described. The construction of a general unifying theory of cationic polymerization is attempted, with peculiarities typical for each system pointed ou