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A radical reaction is a chemical transformation in which an open-shell species bearing one or more unpaired electrons is an essential intermediate, chain carrier, or product-determining species. Radicals can be generated by homolysis, single-electron transfer, hydrogen-atom transfer, photolysis, thermolysis, or redox activation. Their characteristic elementary steps include addition to π bonds, atom transfer, fragmentation, rearrangement, substitution, combination, and disproportionation ^[1]. A chain radical reaction contains initiation, propagation, and termination, whereas a nonchain radical process can proceed through discrete photochemical or electrochemical generation and trapping of each radical. Within radical photochemistry, light creates the radical directly or through excited-state electron or energy transfer ^[2]. The defining criterion is mechanistic participation of an open-shell intermediate, not merely exposure to light or an oxidant. Radical reactions are distinguished from closed-shell ionic and concerted pericyclic reactions by their one-electron bond-making and bond-breaking steps.