This review covers most of the authors' work on ring-opening polymerization and copolymerization of heterocyclic monomers during the time of their cooperation since 1985. The mechanistic aspects of anionic ring opening polymerization of cyclic carbonates with a variety of functional groups are described first. By sequential polymerization of first styrene, methyl methacrylate or suitable heterocyclic monomers and then secondly a cyclic carbonate, the site transformation is highlighted. The influence of the chemical nature of macroinitiators with identical active sites on the course of polymerization of cyclic carbonates was studied for poly(ethylene oxide), poly(tetrahydrofuran), and poly(dimethylsiloxane) mcroiinitiators. For the copolymerization of cyclic carbonates with lactones and lactide the dependence of the polymer microstructure on the polymerization conditions is discussed on the basis of the copolymerization mechanism. The copolymerization of cyclic carbonates with epsilon-caprolactam and with tetramethylene urea results in an alternating copolymer i.e. a poly(ester urethane) and an [m, n]-polyurethane, respectively, the key step being the insertion of the lactam or the cyclic urea into the carbonate chain. The cationic ring opening polymerization of cyclic six and seven membered carbamates leading to [4]- and [5]-polyurethane with uniform microstructure is reported with respect of kinetic, mechanistic, and thermodynamic aspects. This new access to [n]-polyurethanes by a chain growth reaction allows the synthesis of well defined polymer architectures with polyurethane sequences. Sequential polymerization of tetrahydrofuran and the cyclic carbmate with mono- and bifunctional initiators leads to the respective A-B and B-A-B block, copolymers. Site transformation from the oxonium to the immonium active species is the key step in the polymerization mechanism. Finally, mechanistic aspects of the ring-opening polymerization of cyclic ester-amides are presented