Photoinitiated cationic polymerization has not as yet achieved the commercial significance of radical polymerization in important photopolymerization processes, including UV curing and photoimaging. Several reasons for this may be advanced, including: (1) the development of photoinitiators for cationic polymerization was preceded by substantial advances in UV curing technology based on radical polymerization; (2) the early developmental work on UV curing by cationic polymerization utilized aryl diazonium salts as photoinitiators which, while highly photoactive, are thermally unstable, thereby preventing long-term storage of fully formulated compositions; and (3) the discovery of thermally stable onium salts, such as diaryliodonium and triarylsulfonium salts, as effective photoinitiators for cationic polymerization of epoxy-functional resins was made almost simultaneously by several industrial groups, which resulted in an unclear patent situation—a situation which appears, at least in part, to be resolved at present.