Hydrogen is one of the most promising alternative energy resources to replace fossil feedstocks, with so-called “green” hydrogen, derived by water splitting (WS) using renewable electricity or sunlight, the most sustainable. Photocatalytic hydrogen production, in which sunlight is the sole energy input, has been extensively studied, and requires the creation of photogenerated excitons (through irradiation of semiconductors) and their transport to aqueous media. Chemical scavengers, notably electron donating molecules, are widely used to quench photogenerated holes and thereby suppress exciton recombination which otherwise limits the hydrogen yield. Despite their prevalence, the role and significance of such scavengers (also termed sacrificial agents) in photocatalytic WS remains poorly understood, hindering their rational selection. This review focuses on the importance of electron donors in photocatalytic WS, and their participation in the reaction mechanism.