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Ring-opening polymerization (ROP) is a chain-growth process in which a cyclic monomer is cleaved and incorporated into a linear or branched macromolecule, transferring the ring atoms into the polymer backbone. In advanced polymer synthesis the concept is bounded by a cyclic monomer—lactam, lactone, epoxide, cyclic siloxane, cyclic carbonate, or related heterocycle—and by an initiator or catalyst that opens the ring by nucleophilic, cationic, anionic, or coordination-insertion attack. Essential features are a ring-strain or thermodynamic driving force for opening, a propagating chain end that remains competent for subsequent ring opening, and a degree of polymerization controlled by the monomer-to-initiator ratio when the process is living. Controlled ROP of lactide and glycolide exemplifies coordination or activated-monomer mechanisms that yield aliphatic polyesters with defined molecular weight ^[1]. Organocatalytic ROP further shows that small organic acids and bases can open cyclic esters without a metal center, while still preserving the chain-growth kinetic profile ^[2]. ROP is distinguished from ring-opening metathesis polymerization by the absence of an olefin-metathesis elementary step, and from step-growth polycondensation of the corresponding linear monomers by the lack of a small-molecule condensate as a necessary product.