CATIONIC PHOTOINITIATORS are onium compounds that photolyze to form Lewis or Bronsted acids when excited with ultraviolet radiation. In the photolysis process, free radicals are also formed, and therefore, both epoxides and ethylenically unsaturated compounds can be polymerized in a dual-cure process. Although photoinitiators in use today generate Bronsted or protonic acids from iodonium or sulfonium metallic salts, it is important from a historical standpoint to discuss first the Lewis-acid generating or diazonium type photoinitiators.