Mature human milk contains 3%--5% fat, 0.8%--0.9% protein, 6.9%--7.2% carbohydrate calculated as lactose, and 0.2% mineral constituents expressed as ash. Its energy content is 60--75 kcal/100 ml. Protein content is markedly higher and carbohydrate content lower in colostrum than in mature milk. Fat content does not vary consistently during lactation but exhibits large diurnal variations and increases during the course of each nursing. Race, age, parity, or diet do not greatly affect milk composition and there is no consistent compositional difference between milks from the two breasts unless one is infected. The principal proteins of human milk are a casein homologous to bovine beta-casein, alpha-lactalbumin, lactoferrin, immunoglobulin IgA, lysozyme, and serum albumin. Many enzymes and several "minor" proteins also occur. The essential amino acid pattern of human milk closely resembles that found to be optimal for human infants. Possible special functions of milk proteins and enzymes other than as a source of amino acids, are as yet largely speculative. The principal sugar of human milk is lactose but 30 or more oligosaccharides, all containing terminal Gal-(beta 1,4)-Glc and ranging from 3--14 saccharide units per molecule are also present. These may amount in the aggregate to as much as 1 g/100 ml in mature milk and 2.5 g/100 ml in colostrum. Some of them may function to control intestinal flora because of their ability to promote growth of certain strains of lactobacilli. Human milk fat is characterized by high contents of palmitic and oleic acids. the former heavily concentrated in the 2-position and the latter in the 1- and 3-positions of the triglycerides. Fatty acid composition of milk fat varies somewhat with the composition of diet, particularly the fatty acids which it supplies. Phospholipids, amounting in the aggregate to about 75 mg/100 ml, include phosphatidyl ethanolamine, phosphatidyl choline, phosphatidyl serine, phosphatidyl inositol, and sphingomyelin. The principal mineral constituents of human milk are Na, K, Ca, Mg, P, and Cl. Calcium concentrations reported in various studies vary from 25--35 mg/100 ml. Phosphorus at 13--16 mg/100 ml is much more constant but is lower in proportion to casein and calcium than in milks of most other species. Iron, copper, and zinc contents of human milk vary considerably. A long list of other trace elements has been reported. About 25% of the total nitrogen of human milk represents nonprotein compounds including urea, uric acid, creatine, creatinine, and a large number of amino acids. Of the latter, glutamic acid and taurine are prominent. All of the vitamins, except K, are found in human milk in nutritionally significant concentrations.