The cationic ring-opening polymerization of azetidine, N-alkylazetidines and a number of azetidines carrying a second functional group is reviewed. Some of these monomers lead to “living” polymers. The rate constants for the polymerization of different azetidines are discussed and compared with the rate constants for the polymerization of other four-membered heterocycles. The nucleophilic ring-opening polymerization of azetidinium salts is discussed. A new polymer with azetidinium endgroups is described. The synthesis, chemical properties, and applications of a number of polymers with pendant azetidinium substituents are described.