A complex interplay of maternal homeostatic mechanisms influences nutrient transfer to nursing infants, and with a few exceptions, excess maternal intake or a moderate deficiency in the maternal diet does not appreciably alter nutrient transfer to infants unless it has persisted for some time. Milk vitamins D and K contents, even in apparently well-nourished women, may not always provide adequate amounts for infants. Investigations provide evidence that human milk possesses many unique characteristics and that maternal and environmental influences are stronger than previously recognized and appreciated. A complete body of knowledge does not exist to serve as a basis for dietary recommendations to ensure optimal nutrition for mothers and infants. The success of lactation usually is measured in terms of infant performance, and cost and consequence to the mother are seldom considered. Human milk feeding is recommended for the entire first year of life, but few studies focus on the nursing dyad for more than 3 months' duration. Continued study is needed so that nutritional adequacy may be maintained and appropriate dietary guidance can be provided. When human milk feeding is not practiced, modern and reliable data on human milk constituents and their significance to infants also are essential for the preparation of formulas, especially those not based on bovine milk. The adequacy of human milk substitutes cannot be predicted from compositional analysis because of possible differences in compartmentalization and molecular form of nutrients, and such preparations must be evaluated using specific indices of nutrient use, together with traditional anthropometric measures in infants.