Selected topics of current research on HO2/O2- radical reactions in aqueous solutions are described. The subject is introduced by a brief review of the fundamental background chemistry of HO2/O2-, including a description of the rates and mechanisms of formation and decay as well as their spectral properties, current research on self-exchange rates, etc. This is followed by examples of their reactivity with simple metal complexes of iron, manganese and copper and extended to some biological significant metal-containing systems such as porphyrins, haem enzymes and superoxide dismutases. Some recent results on the interaction of these oxy-radicals with antioxidants, specifically vitamin C and E, are discussed. A brief review of some topical chemistry describing HO2/O2- involvement in atmospheric and aquatic chemistry is given. Finally, some new trends in the areas of heterogeneous catalysis and reactor chemistry that involve these species are described.