AA metabolism requires transamination as the first step, generating glutamate and alanine as the major products, followed by oxidative deamination of glutamate with glutamate dehydrogenase (GDH) to form nicotinamide adenine dinucleotide phosphate-oxidase (NAD(P)H), which is converted to ATP. The inborn errors of amino acid metabolism are a family of genetic conditions in which an enzyme deficiency results in the accumulation of a ninhydrin-positive amino acid or a proximal metabolite. They are conceptually identical to disorders caused by enzyme defects that result in the accumulation of the organic acid intermediates. The current chapter strives to highlight the clinical, biochemical, molecular, and pathological features of defects in aromatic amino acid processing and related neurotransmitter metabolism disorders, disorders of glycine metabolism, defects in the processing of sulfur-containing amino acids, disorders of branched-chain amino acid metabolism, proline metabolism, urea cycle disorders, and defects of serine synthesis. Amino acid nitrogen forms ammonia, which is toxic. The liver is the major site of amino acid metabolism in the body and the major site of urea synthesis. The liver is also the major site of amino acid degradation, and partially oxidizes most amino acids, converting the carbon skeleton to glucose, ketone bodies, or CO2. Amino acids are the building blocks of our cellular machinery in the form of proteins and protein complexes. In addition, many important metabolites (i.e., purine/pyrimidines, neurotransmitters etc.) are products of cellular amino acid metabolism.