This chapter discusses the assay of catalases and peroxidases are: (1) catalase assay by disappearance of peroxide; (2) method for crude cell extracts; (3) direct spectrophotometric assay of catalase and peroxidase in cells and tissues; and (4) peroxidase assay by spectrophotometric measurements of the disappearance of hydrogen donor or the appearance of their colored oxidation products. Two methods are described for the catalase assay by disappearance of peroxide are: ultraviolet spectrophotometry and permanganate titration. Ultraviolet spectrophotometryis a method devised, on the basis of the absorption curves for peroxide solutions, for determining the activity of catalase by direct measurements of the decrease of light absorption in the region 230 to 250 mฮผ caused by the decomposition of hydrogen peroxide by catalase. In the case of method for crude cell extracts, oxygen evolution caused by the decomposition of hydrogen peroxide is measured with the conventional manometric technique. Peroxidase assay by spectrophotometric measurements of the disappearance of hydrogen donor or the appearance of their colored oxidation products includes the guaiacol test and the pyrogallol test.