Peroxisomes are cell organelles that perform beta oxidation of very long chain fatty acids. Peroxisomal disorders can be classified into two groups - peroxisomal biogenesis disorders in which the organelles are poorly formed and several functions are affected, and single peroxisomal enzyme deficiencies (at least 10 disorders). Disorders of peroxisomal biogenesis include Zellweger, neonatal adrenoleukodystrophy (nALD), infantile Refsum, hyperpipecolic academia, and classic rhizomelic chondrodysplasia punctata (RCDP type I). All except RCDP type I have elevated C26, and C26/C22 ratio. Single peroxisomal enzyme deficiencies are responsible for X-linked ALD, adrenomyeloneuropathy (AMN), classic Refsum disease, rare forms of RCDP (type II and III), and others. Very Long chain fatty acids are elevated in most disorders of peroxisomal biogenesis and beta-oxidation with the exception of RCDP type I and classic adult Refsum disease. Both of these disorders exhibit elevations in phytanic acid. The VLCFA profile includes C26, C22, C24, C26/C22, C24/C22, phytanic, and pristanic acid.