Ring-opening metathesis polymerization (ROMP) is a chain-growth polymerization in which a metal–alkylidene catalyst opens a cyclic olefin by olefin metathesis and incorporates the ring as a linear unsaturated repeat unit ^[1][2]. The concept is bounded by a cyclic olefin monomer, a metathesis-active alkylidene initiator, and a propagation step that is itself a metathesis event rather than an insertion into a metal–alkyl bond. Essential features are relief of ring strain as a thermodynamic driving force, a (2+2) cycloaddition–cycloreversion sequence at the metal–carbene, and retention of a double bond in each newly formed backbone linkage ^[1]. Living ROMP requires an initiator that enters the propagating state rapidly relative to chain growth and that suppresses chain transfer, so that molecular weight is set by the monomer-to-initiator ratio ^[2]. Macrocyclic monomers enable sequence-controlled ROMP by encoding a defined motif within each ring that is then unrolled into the chain ^[3]. Metal-free ROMP variants generate the metathesis-active species photochemically from an organic initiator, while enol-ether monomers furnish polymers that remain metathesis-derived yet contain hydrolytically labile linkages ^[4]. ROMP is distinguished from ring-opening polymerization of lactones or epoxides by the metathesis rather than nucleophilic or coordination-insertion mechanism, and from acyclic diene metathesis by the cyclic identity of the monomer.