The structure, reaction kinetics and mechanism, substrate specificity, stability and chromogenic, and fluorogenic and chemiluminogenic substrates of peroxidase are reviewed. The applications of peroxidase in the quantitative analysis of hydrogen peroxide, clinically important metabolites via oxidase-coupled reactions, as a label in quantitative immunochemical and DNA hybridization reactions and in immuno-blot and immunohistochemical investigations will also be reviewed.