Inherited disorders of amino acid, organic acid, and peroxisomal metabolism are individually rare but have a high cumulative frequency. The peroxisomal disorders are subdivided into two groups: the peroxisome biogenesis disorders and the single peroxisome enzyme deficiencies. Impaired mitochondrial glutamate transport has been reported in an autosomal recessive neonatal myoclonic epilepsy. Epileptic seizures may be the inaugural or the main symptom in rare inborn errors of intermediary metabolism. Biochemical analyses in Zellweger spectrum disorders show elevated plasma very long chain fatty acids (VLCFA) and usually deficient erythrocyte membrane plasmalogens. Metabolic acidosis with ketosis is suggestive of organic acidurias such as methylmalonic aciduria, propionic aciduria, and isovaleric acidemia, and may indicate a large number of rarer and less known organic acidurias. Therapy with biotin in biotinidase deficiency and with vitamin B6 in vitamin B6-dependency represent two of the most successful therapies for inborn errors of metabolism.