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Olefin polymerization is the catalytic chain-growth conversion of alkenes into high-molecular-weight macromolecules by repeated migratory insertion of monomer into a metal–carbon bond at a coordination-polymerization active site ^[1][2]. The concept is bounded by an insertion mechanism—rather than a radical, cationic, or anionic chain carrier—and by monomers that are simple olefins or α-olefins. Essential components are a transition-metal precursor, a cocatalyst that generates a cationic metal–alkyl and a weakly coordinating anion, and a vacant coordination site that binds the incoming olefin ^[2]. Catalyst classes include metallocenes, ansa-cyclopentadienyl-amido constrained-geometry complexes, and a broad set of non-metallocene ligands on early and late metals ^[1][3][4]. Multinuclear designs place two or more metal centers in proximity so that cooperative insertion or chain transfer can occur ^[4]. The process is distinguished from olefin metathesis polymerization by net saturation of the double bond in each insertion step, and from free-radical polyethylene processes by the metal-centered stereochemical control that determines regiochemistry, tacticity, and copolymer composition. Conceptual limits exclude polymerizations in which the olefin functions only as a comonomer in a radical or ionic mechanism without metal coordination.