Abstract This mini-review provides a brief overview of the role of L-tryptophan in the synthesis of serotonin, quinolinic acid, and kynurenic acid, which are critically important for neuronal development and synaptogenesis, neuroimmune activity, and mitochondrial function. Tryptophan is not synthesised in the human body and it is only available in the diet. As an essential amino acid, it is required for protein synthesis and as a precursor of key biomolecules such as serotonin, melatonin, tryptamine, niacin, quinolinic acid and kynurenic acid, nicotinamide adenine dinucleotide etc. Adequate intake of this amino acid is crucial for growth and development in children and adults. Excessive dietary restriction and malnutrition decreases brain serotonin stores and leads to behavioral changes such as hyperactivity, depression, anxiety, suppression of appetite, anorexia nervosa, and behavioral impulsivity. Dietary supplementation of tryptophan may improve these disorders. Our studies showed that tryptophan isolated from human milk depicted profoundly higher antioxidant activity compared with whole human milk. Tryptophan's powerful antioxidant activity assists in reducing oxidative stress and exerts anti-inflammatory effects. Bioactive factors in human milk are crucial to the health of newborns. Some mothers may not be able to breastfeed their babies. Addition of L-tryptophan to infant formula, equivalent to concentration in breast milk, may significantly improve infant growth and increase antioxidant properties of the formula. Keywords: Tryptophan, Serotonin, Melatonin