After a brief review of the concepts of free radical and enzymatic lipid peroxidation, spontaneous peroxidation (initiation, propagation, termination, end-products) is described in detail on the basis of the latest published studies shedding new light on the role of this process in a wide variety of disease states. Toxic effects of lipid peroxidation and physiologic protective mechanisms are described. Currently available routine methods for evaluating lipid peroxidation, particularly in plasma, although imperfect, are nevertheless valuable in clinical practice in view of the many human disease states involving lipid peroxidation.