X-linked adrenoleukodystrophy is a neurological disease characterized by progressive demyelination with destruction of the white matter, and adrenal insufficiency. Biochemically there is accumulation of very-long-chain fatty acids resulting from an impairment in the peroxisomal oxidation of these fatty acids. In this paper we describe the present state of our knowledge with regard to the organization of the peroxisomal fatty acid oxidation system, with particular emphasis on X-linked adrenoleukodystrophy.