Fatty acids, stored as triacylglycerols, are the main fuel reserves of animals and are readily used by most tissues, with a few specialized exceptions such as nerve cells and erythrocytes. Fatty acid degradation in tissues proceeds mainly through@-oxidation, which involves the successive oxidation removal of acetyl groups from the carboxyl end of long-chain fatty acids. P-Oxidation had been believed to occur only in the mitochondrial matrix in animal tissues until Lazarow and de Duve reported the presence of a 0-oxidation system in rat liver peroxisomes in 1976.'The discovery of peroxisomal, &oxidation is of great importance for the progress of the field of lipid research. Extensive studies on the physiological significance of the system have been undertaken. Studies on the properties of the enzymes in the peroxisomal@-oxidation system are essential to characterize the nature and physiological role of this novel system. In this paper we describe the molecular and catalytic properties of individual enzymes of rat liver peroxisomes.