The story of the discovery and development of onium salt photoinitiators for cationic polymerization is chronicled. The chemistry of the synthesis of these compounds is outlined, and the mechanisms of their initiation are discussed briefly. Among the most useful of these types of photoinitiators are diaryliodonium and triarylsulfonium salts, which are used widely for photoinduced cationic crosslinking reactions. From the very beginning, onium salt photoinitiated cationic polymerizations have found use in a multitude of practical applications. Specifically discussed in this article are the use of onium salts in coatings, adhesives, printing inks, release coatings, stereolithography, holographic recording, photocurable composites, and microelectronic photoresists.