This review will first give a breif description of coloured dissolved organic matter (CDOM) from the aquatic environment or extrated drom soil. Data on chemical structure and spectroscopic properties are presented and the major photochemical processes involving CDOM as well as the main reactive species produced upon their irradiation are described. The fourth part of this chapter is an overview of studies related to the phototransformation of organic contaminants in the presence of CDOM, either under field or laboratory conditions. CDOM was found to be able to photo-induce the transformation of numerous organic pollutants. Hydroxyl radicals play a minor role, probably because they are efficiently scavenged by the CDOM itself. Examples of the oxidation of pollutants by singlet oxygen and by reactive triplet states are given, as are examples of photoreduction.