Lactogenesis begins during pregnancy and secretory material accumulates in the acini from the third month of gestation. Milk production is stable during the first months of lactation, but there is a wide range of milk intake among healthy breast-fed term infants, averaging 750 to 800 ml per day but ranging from 450 to 1200 ml/day because of infant demands. Human milk fat content is the main source of energy and its most variable constituent. Fat content is low in colostrum and increases from 2% to 5% in mature milk. Dietary fats are the main source of infants' energy, provide essential fatty acids, and facilitate the absorption of fat-soluble vitamins. Immunological and anti-infectious proprieties of human milk are of major importance compared with formulas. Environmental factors, including early infant nutrition, may influence their development. Potential energy mobilization during lactation depends on weight gain during gestation and nutritional status of the mother.