Peroxisomes, subcellular organelles found in nearly all eukaryotic cells, are involved in numerous biochemical functions within the cell. There has been an increasing understanding of the genetic mechanism of the diseases of the single peroxisomal enzyme abnormalities as well as defects of peroxisome biogenesis. Peroxisome assembly disorders including Zellweger syndrome and rhizomelic chondrodysplasia punctata are caused by genetic defects in PEX genes and the altering of their proteins, peroxins, which are necessary for the importation of targeted proteins into the peroxisomes. Therapies for peroxisomal disorders have been unsatisfactory to date, but there has been interest in docosahexaenoic acid in assembly disorders and phenylbutyrate and lovastatin in adrenoleukodystrophy (ALD). Whether any of these therapies will result in clinical improvement awaits additional study.