Interest in the utility of polyester, i.e. polylactide, as a commodity polymer has increased significantly in past decade due to numerous environmental advantages over conventional petrochemically derived plastics. This review article summarizes recent developments of ringopening polymerization of lactides using metal complexes as catalysts/initiators. These complexes described here are classified according to metal groups in the periodic table, including Group 1-2 and zinc. Most attention is devoted to the well-defined metal complexes. Where useful properties, such as high reactivity or controlled property, have been observed a more detailed examination of the initiator is provided.