Photoredox catalysis is a catalytic mode in which a light-absorbing catalyst, upon excitation, engages in single-electron transfer with an organic substrate or a sacrificial redox partner, thereby generating radical or radical-ion intermediates under conditions that would not produce those intermediates thermally ^[1][2]. In radical photochemical reactions the concept is bounded by a closed photoredox cycle: the photocatalyst must be restored to its original oxidation state after the productive electron transfer. Essential features are a chromophore with a sufficiently long-lived excited state, a pair of excited-state redox potentials that match the substrate, and a subsequent radical transformation—addition, coupling, or fragmentation—of the photogenerated intermediate ^[1][3]. Catalysts include transition-metal polypyridyl complexes, especially iridium and ruthenium species, and organic dyes that operate by analogous electron-transfer cycles ^[1][2]. Dual catalytic systems merge the photoredox cycle with a second cycle, such as organocatalysis or nickel catalysis, so that the radical intermediate is intercepted in a controlled bond-forming step ^[4]. The concept is distinguished from direct photochemistry of the substrate, in which no catalytic chromophore mediates the electron transfer, and from energy-transfer photocatalysis, in which the excited catalyst transfers energy rather than an electron.