Defects in the synthesis pathways of amino acids have only been discovered in recent years and are relatively unknown to most clinicians. In this chapter disorders neurological symptoms caused by genetic defects in the synthesis of the amino acids serine, glutamine and proline are discussed. Patients with serine deficiency invariably suffer from severe neurological symptoms such as microcephaly, psychomotor retardation and intractable seizures. The same is true for patients with a defect in the synthesis of glutamine who present with hypotonia and epileptic encephalopathy.Patients with a disorder in the synthesis proline have mental retardation in combination with symptoms of connective tissue disease. All amino acids synthesis disorders, except for one of the proline defects, can be diagnosed by routine amino acid analysis. Given the fact that the disorder discussed here lead to deficiencies of amino acids opens up possibilities to treat these patients with the amino acids that can't be synthesised.Prompt recognition and treatment of amino acid synthesis disorders are of outmost importance to obtain satisfactory treatment results.