The three peroxisome proliferator-activated receptors (PPARs), PPAR [alpha],[delta] and [gamma], form a subfamily of the nuclear hormone receptor gene family. PPAR [alpha] has been shown to bind and be activated by leukotriene B4 and fibrates, whereas prostaglandin J2 derivatives and the antidiabetic thiazolidinediones, respectively, are natural and synthetic ligands for PPAR [gamma]. The availability of ligands and activators for PPAR [alpha] and PPAR [gamma] allowed an initial assessment of their respective functions. PPAR [alpha] and PPAR [gamma] are shown to function as important regulators in lipid and glucose metabolism, adipocyte differentiation, inflammatory response and energy homeostasis. PPAR [alpha] seems to mediate its pleiotropic effects mainly through the stimulation of oxidation of lipids, whereas PPAR [gamma] is a key mediator of lipid storage. The next few years will be very exciting as additional studies will refine our current knowledge about PPAR [alpha] and PPAR [gamma] and may reveal a ligand and role for the lonesome orphan among the PPARs, PPAR [delta].