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Chain-growth polymerization initiated and propagated by a positively charged active center is cationic polymerization. The chain carrier is a carbocation, an oxonium ion, or a related electrophilic species, and the monomers are electron-rich alkenes (vinyl ethers, N-vinylcarbazole, styrenes with donor substituents) or heterocycles that open to cationic ends. Living cationic polymerization is realized when reversible deactivation or a dormant–active equilibrium suppresses transfer and termination, allowing molecular-weight control and block-copolymer synthesis ^[1]. Visible-light control of vinyl-ether cationic polymerization modulates the concentration of the ionic chain end without changing the cationic nature of propagation ^[2]. Catalyst-controlled stereoselective cationic polymerization of vinyl ethers shows that the same electrophilic mechanism can encode tacticity when the counterion and Lewis acid are suitably paired ^[3]. The concept is the charge-inverted counterpart of anionic polymerization and is distinct from radical polymerization and from coordination insertion at a neutral metal alkyl. Photoinitiated and photocontrolled cationic processes remain cationic polymerizations so long as the propagating species is positively charged.