The F2 isoprostanes are a unique series of prostaglandinlike compounds formed in vivo from the free-radical-catalyzed peroxidation of arachidonic acid independent of the cyclooxygenase enzyme. The purpose of this review is to briefly summarize the status of our current knowledge regarding the isoprostanes. Novel aspects related to the biochemistry of isoprostane formation and methods by which these compounds are analyzed are discussed. The utility of measuring F2 isoprostanes as markers of lipid peroxidation in animal models of oxidant stress are then outlined. Finally, examples are provided in which quantification of isoprostanes have illuminated the role of oxidative injury in several human diseases.