Recent work on lipid classes and the fatty acid composition of milk is reviewed. At least 98% of the lipids are triacylglycerols with about 1% phospholipids and 0.3–0.4% cholesterol. Desmosterol has been identified. The major phospholipids (% of total) are: sphingomyelin (32), phosphatidylcholine (25), and phosphatidyl-ethanolamine (28). The 12:0 content of colostrum is lower than that of mature milk. The 18:2 content of mature milk can be altered by diet; otherwise, the fatty acid composition is remarkably constant. Long-chain polyunsaturates appear to be needed for development of the infant's brain and nervous system. Data on the contents of these acids are given. The fat-soluble vitamins have been analyzed by high-performance liquid chromatography. The amounts of vitamins D and K in milk, unlike those of A and E, are lower than the quantities required to meet the U.S. Recommended Daily Allowances. Vitamin D sulfate does not appear to be a significant antirachitic component of human milk. Nutritional and analytical aspects are emphasized throughout and areas for future investigations indicated.