In this paper, we present case studies of oxidative transformations of contaminants by oxidants which are relevant in natural and in technical systems. These oxidants are reactive oxygen species (ROS), manganese(III,IV), iron(III), and molecular dioxygen (O-2). Regarding ROS, we discuss i) their concentrations and their efficiencies as oxidants in natural waters and in water treatment, ii) reactions occurring in bromide-containing waters in the presence of ROS, iii) role of iron in the formation of ROS, and iv) quantitative structure-activity relationships (QSAR) of reactions of ROS with contaminants. Concerning Mn-III and Mn-IV as oxidants, we present experimental studies on the oxidation of anilines by delta-MnO2. With respect to oxidative transformation of the hydrophilic organic contaminants ethylenediaminetetraacetate (EDTA) and nitrilotriacetate (NTA), we show that these organic complexing agents are efficiently oxidized by Fe-III and O-2, respectively, if catalyzed by light (for the oxidation of EDTA by Fe-III) and by enzymes (for the oxidation of NTA by O-2).