Hydrogen atom transfer (HAT) reactions are pivotal in chemical and biochemical processes. In contrast to electron transfer and energy transfer mechanisms, HAT offers a distinctive advantage by overcoming the constraints of redox potentials, thereby activating substrates under mild conditions in organic transformations. HAT in photocatalysis has two approaches: indirect hydrogen atom transfer (i-HAT) and direct hydrogen atom transfer (d-HAT). In the i-HAT process, a photocatalyst is excited upon light absorption to generate radical or radical ion species as hydrogen abstractors through electron transfer events, for example, reduction of persulfate (S2O8 2-) to a sulfate radical anion (SO4 •-) and oxidation of thiol (R-SH) to thiyl radical (R-S • ). In comparison, the d-HAT process directly abstracts a hydrogen atom from a substrate molecule by an excited photocatalyst, forming a radical intermediate and the H • -doped form of the photocatalyst (PC • -H) (Figure ).