Synthetic DL-methionine (DLM) supplements poultry diets to enhance production. The bioefficacy of liquid methionine is generally lower than that of powder methionine; however, if the level of total sulfur amino acids (TSAA) is set to the commercial recommendation, the bioefficacy of liquid methionine seems to be equal to that of powder methionine (equimolar basis). Absorption and transportation in the segment of the jejunum differ between liquid and powder methionine because multiple systems are involved. Methionine supplementation in a low-protein diet alleviates the negative effects of heat stress. The supplementation improves the amino acid balance and consequently promotes growth performance by enhancing feed efficiency, increases protein synthesis and decreases fat synthesis. Methionine supplementation also improves the immune response through direct effects (protein synthesis and breakdown) and indirect effects (derivatives of methionine). As various factors influence the methionine requirement, the requirements of commercial strains are higher than those recommended by NRC (1994). Moreover, the methionine requirement expressed as a percentage of diet declines during the starter and grower phases, while the requirement related to lysine is little changed (tends to increase).