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Light-driven generation of electrophilic initiating species that then propagate a cationic chain-growth process defines cationic photopolymerization. A photoinitiator—commonly a diaryliodonium or triarylsulfonium salt, or a sensitizer that reduces such an onium salt—produces a Brønsted or Lewis acid upon irradiation; that acid initiates ring-opening of epoxides, oxetanes, or vinyl ethers ^[1]. Propagation proceeds through oxonium or carbocationic chain ends and is not quenched by oxygen, which distinguishes the process from radical photopolymerization. Wavelength can be extended from the ultraviolet into the red by perylene and related sensitizers that still generate the cationic initiator ^[2]. Chemiluminescence can replace external irradiation when a sulfonium salt is activated by a chemical light source, but the initiating cation and the subsequent cationic propagation remain the same ^[3]. The concept is bounded as photoinitiated cationic chain growth: thermal cationic polymerization lacks the photochemical initiation step, and radical photopolymerization uses neutral radical rather than cationic active centers.