A report on breast-feeding and lactation is presented. Breast-feeding physiologically depends on the development of milk-producing tissues in the breast during pregnancy, the initiation of milk production, the maintenance of milk production, and milk ejection (let-down reflex). Prolactin and human growth hormone are of primary importance in the hormonal complex controlling the processes during lactation. If the let-down reflex fails, lactation soon ceases. Most breast-feeding problems are psychological in nature. The failure of the let-down reflex, nipple abnormalities, mastitis, and breast engorgement impede breast-feeding and milk removal. In undernourished women with inadequate nutritional stores developed during pregnancy, breast-feeding may deplete maternal tissues, with possibly serious consequences for maternal health. A nursing mother should increase her fluid, caloric, and protein intake. Maternal milk can satisfy the total nutritional needs of the infant during the first 4-6 months of life and can also help protect against polio, staphylococcus infection, and bacteria-caused diarrhea. The practice of breast-feeding is declining all over the world, especially in urban areas, and among the poor infant mortality, morbidity, and malnutrition rises. Breast-feeding reduces the chances of conception during postpartum amenorrhea. Anovulation lasts for about 7 months, on the average, in lactating women. Lactating women have a considerably longer period of postpartum amenorrhea than do nonlactating women. A definite effect of the length of lactation on the birthrate has been demonstrated. It appears that the desire not to breast-feed has as much an effect on milk quality and the duration of lactation as the estrogen content of oral contraceptives. Estrogen has only a minimal effect on milk composition. Progestogen-only contraception does not seem to have adverse effects on the quantity or quality of milk and may, in some women, increase the volume of milk and extend the duration of lactation. Thyrotropin-releasing hormone therapy, which increases milk production and acts as a contraceptive, apparently has no adverse metabolic or endocrine effects on the lactating mother or infant. The importance of breast-feeding and its implications for family planning programs should be carefully considered.