The prostanoids (prostaglandins, prostacyclins, thromboxanes) are one subset of arachidonic-acid derived lipid mediators of inflammation. The leukotrienes constitute the second major class of arachidonic-acid derived lipid mediators. A variety of inflammatory signals (including exogenous mediators such as endotoxin/lipopoly-saccharide and endogenous mediators such as inflammatory cytokines) stimulate cellular phospholipase activities. As a result, arachidonic acid is released from phospholipid membrane stores. This free arachidonic acid is converted either (i) to leukotrienes via the lipoxygenase pathway(s), or (ii) to prostanoids. The key enzyme in prostanoid biosynthesis is prostaglandin synthase (PGS), also known as cyclooxygenase (COX). PGS/COX converts free arachidonate first to prostaglandin G2 (PGG2) by a cyclooxygenase reaction, and then -in a second enzymatic step- to prostaglandin H2 (PGH2) by a hydroperoxidase reaction. PGH2 is then converted to the various prostaglandins (PGE2, PGD2, PGF2a, etc), prostacyclin, (PGI2) or thromboxane by cell-type restricted synthetases that use PGH2 as substrate. The various prostanoids have a wide range of positive and negative immuno-modulatory effects on lymphoid, myeloid, and endothelial cells. New pharmacologic agents that modulate the activity of enzymes involved in prostanoid synthesis or the expression of the genes encoding these enzymes will provide improved directions in the management of acute and chronic inflammatory diseases.