The free radical reaction of polyunsaturated fatty acids with molecular oxygen leads to hydroperoxides as the first stable products. From linoleic acid the two conjugated diene hydroperoxides at carbons 9 and 13 were considered the only primary products until the recent discovery of the bis-allylic 11-hydroperoxide. The 11-carbon is the site of the initial hydrogen abstraction, and the 11-hydroperoxide is formed without isomerization of the 9,10 and 12,13 cis double bonds. In the autoxidation reaction, bis-allylic hydroperoxides are obtained only in the presence of an efficient antioxidant, for example, alpha-tocopherol. The antioxidant functions as a hydrogen atom donor, necessary to trap the fleeting bis-allylic peroxyl radical intermediate as the hydroperoxide. Understanding of the mechanism of formation of bis-allylic hydroperoxides has led to increased appreciation of the central role of the intermediate peroxyl radical in determining the outcome of lipid peroxidation.