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Photooxidation is an oxidation whose initiating or driving event is absorption of light by the substrate, a sensitizer, or a photocatalyst. The excited species can transfer energy to triplet oxygen to generate singlet oxygen, transfer an electron to produce radical ions and superoxide, or generate a semiconductor hole that removes an electron from an adsorbed substrate. A net increase in substrate oxidation state and a necessary photonic step define the concept; dark autoxidation and photochemical reactions with no oxidative change are excluded. In sensitized photooxygenation, singlet oxygen reacts with organic π systems through ene reactions, (4+2) cycloadditions, or (2+2) pathways, and it can oxidize sulfides and phosphines ^[1]. Other photooxidations proceed by electron transfer and radical chains rather than singlet oxygen ^[2]. Photooxidation is broader than photooxygenation because oxygen-atom incorporation is not required, and it differs from photoredox catalysis because catalyst turnover is not part of every photooxidation.