Isoprostanes are a family of prostaglandin-like compounds that are generated in vivo by free radical attack of esterified arachidonic acid and then released in free form in biological fluids. Since their discovery in 1990, they have been extensively used as biomarkers of lipid peroxidation and oxidative damage in an increasing number of human diseases. Few members of the isoprostane family are biologically active and could contribute to the functional consequences of oxidant injury. The present review summarises the current knowledge on formation and biological activities of these lipid peroxidation products, focusing on their role as valuable biomarkers to investigate the involvement of oxidative stress in the pathogenesis of infant and adult central nervous system diseases. In addition to isoprostanes, a new class of free radical-mediated peroxidation products, named neuroprostanes, is discussed. Neuroprostanes derive from peroxidation of docosahexaenoic acid, a polyunsatured fatty acid particularly abundant in neurons, and may represent a more selective index of brain oxidant injury than isoprostanes. In spite of some discrepancies in the results reported in different studies, isoprostane and neuroprostane levels in human biological fluids, as well as in experimental models of brain diseases, appear to be valuable indicators not only to monitor the occurrence and the causal role of oxidative stress in brain pathologies, but also for critical selection and evaluation of appropriate antioxidant therapies.