This chapter discusses disorders of 8 different amino acids (phenylalanine, tetrahydrobiopterin, tyrosine, methionine, glycine, tryptophan, lysine, and serine). Creatine deficiency syndromes, Sulfite oxidase and Molybdenum cofactor deficiency, defects of GABA metabolism and γ-Glutamyl Cycle have been included in this chapter as well. Although it is not an amino acid disorder, oculocerebrorenal syndrome (OCRL) (Lowe syndrome) remains in this chapter because it was included here in the first edition of the book. I summarize the biochemical abnormalities and pathology for each condition, and then discuss the clinical findings, diagnosis and treatment. For some of these disorders detected through newborn screening, early the morbidity and mortality in patients. For others, there are not yet any effective treatment options.