In the last decade, there has been increased interest in lipase/esterase-catalyzed ring-opening polymerization as an alternative to metal-based catalytic processes. This review focuses on three components in the reaction system, namely biocatalysts, reaction medium and monomers. Novel lipases or esterases are described with particular emphasis on, those derived from thermophiles, immobilized enzymes and recombinant whole-cell biocatalysts. Green solvents in enzymatic ring-opening polymerization, including water, ionic liquids, supercritical carbon dioxide and hydrofluorocarbon solvents, are also discussed. Enzymatic ring-opening polymerization is reviewed with regard to the variety of polymers obtainable, such as polyesters, polycarbonates, polyphosphates and polythioesters. Among these, enzymatic synthesis of polyesters has been most widely investigated, and is discussed for lactones with small to large ring sizes. Finally, the mechanism of enzymatic ring-opening polymerization is described, which is generally accepted as a monomer-activated mechanism. Overall, the review demonstrates that lipase/esterase-catalyzed synthesis of polymers via ring-opening polymerization provides an effective platform for conducting "green polymer chemistry". (C) 2011 Elsevier Ltd. All rights reserved.