The isoprostanes are a unique series of prostaglandin-like compounds formed in vivo from the free radical-catalyzed peroxidation of arachidonic acid independent of the cyclooxygenase enzyme. The purpose of this article is to summarize our knowledge regarding the isoprostanes and discuss what are avenues for future research. Novel aspects related to the biochemistry of isoprostane formation and methods by which these compounds are analyzed, including potential pitfalls that may occur during the analysis, are discussed first. The isoprostanes possess potent biological activity, and their potential role in mediating certain aspects of the detrimental effects of oxidant stress is then examined. A considerable portion of this review deals with the utility of measuring isoprostanes as markers of oxidant injury both in vitro and in vivo. A number of studies have shown these compounds to be extremely accurate markers of lipid peroxidation in animal models of oxidative stress and have illuminated the role of oxidant injury in association with a number of human diseases.