Nucleophilic initiation of a chain-growth process in which the propagating species is an anion defines anionic polymerization. Typical initiators are organolithium reagents, alkali metal aromatic radical anions, or alkoxides; the monomer is an electron-deficient alkene, a strained heterocycle, or a related unsaturated electrophile. In the living limit, termination and transfer are absent, so the number-average molecular weight is set by the monomer-to-initiator ratio and the chain end remains available for further monomer addition or end-capping ^[1]. Ring-opening anionic polymerization of β-lactam sugar monomers produces poly-amido-saccharides, showing that the same ionic mechanism applies to cyclic amides ^[2]. Living anionic polymerization of phosphaalkenes likewise affords homopolymers and block copolymers with controlled chain length from a P=C monomer ^[3]. Architectural complexity—stars, combs, and cyclic polymers—arises from multifunctional initiation or from coupling of living carbanionic ends ^[1]. The concept is distinct from cationic polymerization (positively charged active centers) and from radical polymerization (neutral odd-electron chain ends).