Discussion in this paper focuses on methods for prediction of one important transformation process, photolysis by the action of sunlight. Two general classes of photochemical transformation are direct photolysis, initiated by direct absorption of light by the pollutant, and indirect or sensitized photolysis involving light absorption by natural 'photosensitizers.' At concentrations normally encountered in natural waters, direct photolysis is described by first order rate expressions. Equations that employ solar irradiance, quantum yields, and absorption spectra of pollutants are used to predict photolysis rates as a function of time of day, season, location, and water depth. Sensitized photolysis can proceed by a variety of mechanisms including energy transfer, sensitizer-pollutant reaction, and oxidation involving singlet oxygen or free radicals. Recent studies have indicated that singlet oxygen, a species that rapidly oxidized certain pollutants such as polycyclic aromatics, is generated photochemically in a variety of natural waters.