Linoleic acid is an unsaturated fatty acid that is an essential fatty acid in mammals because they cannot synthesize double bonds in fatty acids beyond position #9. Linoleic acid and linolenic acid are thus essential in mammalian diets, since they have double bonds beyond position #9 (at positions 9,12 and at positions 9,12, and 15 for linoleic and linolenic acid, respectively).

Linoleic acid is an important precursor of arachidonic acid (Figure 18.33), which is itself a precursor of the eicosanoids, a class that includes the prostaglandins and leukotrienes.