Lipids, in addition to serving as fuel stores and structural components of cell membranes, act as effectors and second messengers in a variety of biological processes including those associated with the immune system. These lipid mediators and regulators differ in structural composition and exert a diverse array of effects on cellular functional activities including those linked to homeostasis, immune responsiveness, and inflammation. They function as intercellular mediators and at the intracellular level act as critical conduits of external stimuli in signal transduction cascades. Lipid derived messengers and their receptors also may interact with other signaling molecules. Exogenous compounds such as cannabinoids share functionally relevant receptor binding domains with those for endogenous lipid signaling ligands and have the potential to alter transductional cascades linked to immune functional activities.