The amount of human milk ingested by the nursing infant is about 600 ml per day. The average lipid content of the mature ranges from 3.2 to 3.5% and the amount does not appear to be influenced by diet. About 98% of the lipid is triacylglycerol in which most of the secondary ester is palmitic acid, a unique structure possibly responsible for the relatively high absorbability of the fat. Small quantities of other lipids are present. Amounts of cholesterol reported, range from 200 to 564 mg per 100 g of lipid. While 167 fatty acids have been positively and tentatively identified as being present in human milk lipids, the major fatty acids are palmitic, stearic, oleic, and linoleic. The composition can be changed by diet, which linoleic acid contents of from 1.0 to 45.0% having been found. The "average" linoleic acid is about 10% and this amount is apparently adequate for the essential fatty acid requirements of the infant. The quantity of vitamin E also appears to be satisfactory. The hypothesis that a cholesterol challenge to the breast fed infant would enable the adult to more efficiently metabolize the sterol does not seem to be supported by available evidence, primarily, because the cholesterol content of human milks varies so markedly; 26 to 52 mg per 8 ounces. The compositions of most infant formulas currently in use in the United States are presented for comparison and convenience and a few possible problems associated with their consumption are discussed. We have suggested several areas where, in our opinion, additional research would provide useful information.