Although peroxisomes were initially believed to play only a minor role in mammalian metabolism, it is now clear that they catalyse essential reactions in a number of different metabolic pathways and thus play an indispensable role in intermediary metabolism. The metabolic pathways in which peroxisomes are involved include the biosynthesis of ether phospholipids and bile acids, the oxidation of very long chain fatty acids, prostaglandins and unsaturated long chain fatty acids and the catabolism of phytanate and (in man) pipecolate and glyoxylate. The importance of peroxisomes in cellular metabolism is stressed by the existence of a group of inherited diseases, the peroxisomal disorders, caused by an impairment in one or more peroxisomal functions. In the last decade our knowledge about peroxisomes and peroxisomal disorders has progressed enormously and has been the subject of several reviews. New developments include the identification of several additional peroxisomal disorders, the discovery of the primary defect in several of these peroxisomal disorders, the recognition of novel peroxisomal functions and the application of complementation analysis to obtain information on the genetic relationship between the different peroxisomal disorders. The peroxisomal disorders recognized at present comprise 12 different diseases, with neurological involvement in 10 of them. These diseases include: (1) those in which peroxisomes are virtually absent leading to a generalized impairment of peroxisomal functions (the cerebro-hepato-renal syndrome of Zellweger, neonatal adrenoleukodystrophy, infantile Refsum disease and hyperpipecolic acidaemia); (2) those in which peroxisomes are present and several peroxisomal functions are impaired (the rhizomelic form of chondrodysplasia punctata, combined peroxisomal beta-oxidation enzyme protein deficiency); and (3) those in which peroxisomes are present and only a single peroxisomal function is impaired (X-linked adrenoleukodystrophy, peroxisomal thiolase deficiency (pseudo-Zellweger syndrome), acyl-CoA oxidase deficiency (pseudo-neonatal adrenoleukodystrophy) and probably, the classic form of Refsum disease.