Oxidant stress--i.e. the prevalence within the cell of oxidizing species over the cellular antioxidant potential--is recognized as a primary factor in the pathogenesis of a series of important human pathologies. The possibility of a preventive or therapeutic intervention by means of antioxidant factors, including those naturally contained in diet, increases the importance of a correct understanding of the molecular mechanisms involved in such pathologies. A number of sophisticated biochemical methods are available, for the monitoring of nearly all oxidant stress-related processes; their applicability to in vivo conditions is however limited. In these conditions, the histochemical visualization in tissue sections and isolated cells of selected molecular markers for oxidative phenomena can often provide valuable information about the distribution of processes in vivo. This paper includes an overview of the basic biochemical reactions occurring during oxidant stress and lipid peroxidation, as well as of the main histochemical studies published in the field.