Mammalian peroxisomes are subcellular organelles involved in the metabolism of hydrogen peroxide (oxidases, catalase), lipid anabolism (ether lipid biosynthesis) and catabolism (oxidation of fatty acids and fatty acid derivatives), and intermediary metabolism (transaminases, dehydrogenases). Peroxisomes are formed by division, as is the case for mitochondria, but, in contrast to these organelles, they do not contain DNA. They were discovered and characterized (by biochemical and morphological techniques) later than the majority of the other cell components and specific procedures have been developed for their isolation. Functions of peroxisomes are, as a rule, shared by other cell compartments so that specific enzyme assays have also been developed. Combination of specific isolation procedures, enzyme assays and morphological analysis have resulted in our current knowledge of peroxisomal physiology which, however, has greatly benefited, as in the case of lysosomes, from the study of inborn errors of metabolism and the contribution of molecular biology. Novel enzymes and metabolic pathways have been demonstrated to exist in peroxisomes and human genetic disorders affecting one or several of these functions have been recognized.