Leukotrienes (LTs), prostaglandins, and thromboxanes are products of the oxidative metabolism of arachidonic acid (AA) in mammalian cells. LTs are linear unsaturated C20 fatty acids carrying polar substituants (one or more oxygenated groups and a peptidic group), in contrast with eicosanoids derived from the cyclooxygenases which are cyclized oxygenated derivatives of C20-unsaturated fatty acids. LTs are further characterized by the presence of three conjugated double bonds (conjugated triene) which account for the typical UV absorption pattern of this novel family of compounds. The discovery of the leukotriene pathway was closely associated to biochemical and pharmacological studies on the “slow reacting substance of anaphylaxis” (SRS-A) and the reader should refer to reviews for detailed reports on the structure elucidation of SRS-A 1,2 and the LTs, 3,6 and to other chapters dealing with the biological and pharmacological properties of these putative mediators of inflammation and allergy.