The measurement of milk yield in any suckling mammal is difficult, and studies on women pose additional restrictions because of the limitations on the nature of the techniques which can be used. The most commonly used method for determining milk yield in women is test weighing. In this method either the infant or the mother is weighed before and after a breast feed, and the difference between the two weights is considered to be the amount of milk produced. The reliability of the results obtained by the test weighing procedure depends on both the interval over which milk yield is recorded and the pattern of breast-feeding adopted by the mother. Other methods including isotope dilution, infant growth, and milk expression have been used to determine milk yield in women. Estimations of the yield of breast milk are usually made either to determine the nutrient intake of the infant or to investigate the synthetic capacity of the lactating breast. These goals require the estimation of both milk yield and milk composition. Although it is difficult to obtain a reliable measure of the average fat content of milk consumed by an infant, estimates can be obtained by the use of either a nipple-shield sampling system or a predictive equation based on the concentration of fat in fore- and hindmilk.