Advanced oxidation processes (AOPs) have been used to remove recalcitrant organic pollutants in water and wastewater. AOPs rely on the formation of reactive radicals, such as hydroxyl radical, sulfate radical and superoxide radical, for the superior degradation efficiency of target pollutants. In this presentation, recent works on ozonation, catalytic ozonation, Fenton-like process and persulfate technology conducted in our research group will be summarized. The focus will be on fundamental chemistry and reaction mechanisms. Specifically, the roles of natural organic matter (NOM) and effluent organic matter (EfOM) in terms of direct ozone reaction, initiation, promotion and inhibition in the OH radical chain reactions during ozonation were explored. The use of multiwall carbon nanotube (MWCNT) in transforming molecular ozone to hydroxyl radical for the removal of emerging contaminants was investigated. The integration of MnO2 with H2O2 in a Fenton-like process was examined for the degradation of perfluorooctanesulfonic acid (PFOS) in neutral pH, in which the roles of formed superoxide radical and hydroxyl radical were investigated. Finally, CuO was used to activate peroxydisulfate (PDS) for the removal of chlorophenol. Surface- bound sulfate radical and hydroxyl radical were found to be responsible for the chlorophenol degradation and the kinetics was successfully modeled.