Agrochemicals are introduced into aquatic environments through direct application or through indirect pathways such as runoff, partitioning into condensed phases in the troposhere, and atmospheric deposition into freshwater and marine systems. Sunlight-induced direct and indirect photoreactions are an important sink for agrochemicals in such environments. Both laboratory and field studies have been conducted to develop and test concepts and mathematical relationships that describe environmental photoreactions. These efforts achieved early success in the case of photoreactions that involve the direct absorption of solar radiation by agrochemicals. Direct photoreactions, however, account for only a part of these sunlight-induced reactions. Recent studies have shown that other reactions involving photochemically produced reactive chemical transients also are an important fate for many agrochemicals. These include hydroxyl, hydroperoxyl/superoxide, organoperoxyl, carbonate, and dibromide ion radicals as well as singlet molecular oxygen and solvated electrons. Hydrogen peroxide, a widely distributed oxidant in the sea, fresh waters, and cloud water that is produced via the intermediacy of superoxide radicals, may be involved in the oxidation of agrochemicals by the Fenton reaction or by peroxidase-catalyzed oxidations.