Publisher Summary This chapter reviews eicosanoid actions in insect immunity. Eicosanoids are oxygenated metabolites of arachidonic acid and two other C20 polyunsaturated fatty acids. Groups of eicosanoids include prostaglandins, lipoxygenase products, and epoxyeicosatrienoic acids. Eicosanoids somehow act in the immune functions responsible for clearing injected bacteria from hemolymph circulation in tobacco hornworms, Manduca sexta. They also mediate some or all of the cellular mechanisms responsible for clearing bacterial infection from hemolymph circulation. Several research groups considered the eicosanoid hypothesis and experimental results strongly support the hypothesis for at least 20 species representing seven orders. If eicosanoids mediate cellular immune reactions, insect immune tissues must be able to biosynthesize eicosanoids. It is safe to say that insects are generally competent to produce PGs and other eicosanoids. Aside from documenting the presence of eicosanoid systems in insect tissues, two studies suggest a direct link between eicosanoid biosynthesis and immune signaling. Hemocytes are also able to migrate toward the source of a bacterial peptide and to demonstrate that the chemotactic response depended on eicosanoids. Eicosanoids may act in other aspects of insect antiviral immunity as well, because recent work indicates that oral administration of EBIs increased susceptibility of gypsy moth larvae to Lymantria dispar nucleopolyhedrovirus. New information on possible roles of eicosanoids in insect–virus interactions and research into the mechanisms of eicosanoid action are among the visible frontiers of the knowledge available on insect immunology.