Photochemical processes in surface water induced by sunlight affect the redox chemistry of these waters. The humic substances (HS) are almost half of the dissolved organic carbon in natural waters and represent the main fraction of colored dissolved organic carbon that absorbs solar radiation. Upon UV-visible light absorption, the excited states of HS interact with species present in natural waters to form reactive intermediates, such as: singlet oxygen O₂(¹Δ_g), hydroxyl radical (HO^•), superoxide radical anions (O₂^•-) and organic peroxides. In the presence of inorganic ions, additional reactive species are produced. Furthermore, experimental data indicate that the photoionization of HS yield solvated electrons (e^-_aq.). These reactive intermediates are responsible for the degradation of organic pollutants and dissolved organic matter. This chapter reviews:  1) recent research concerned with the characterization of HS from different origins; 2) the ability of HS to generate reactive species by photosensitization, 3) the reactivity of the reactive species with the HS and 4) the degradation of pollutants photosensitized by HS. General results about the photodegradation of contaminants are reviewed, but special attention is paid to that of two model compounds. These are clomazone and trichloroacetic acid (TCAA). The herbicide clomazone inhibits synthesis of chlorophyll and carotenoids in sensitive plants, resulting in foliage devoid of pigmentation. Trichloroacetic acid is highly phytotoxic, and is partly responsible for conifer forest dieback due to its accumulation in the tree needles and in soils. There is abundant evidence that soil microorganisms dehalogenate TCAA. However, removal by oxidative waste water treatment methods is only effective at temperatures over 40°C, if vacuum-UV photolysis of water is applied, and it cannot be totally removed by UVC photolysis. For these reasons, photosensitized degradation of clomazone and TCAA is of environmental interest.