The biological role of lipid peroxidation products has continued to receive a great deal of attention not only for the elucidation of pathological mechanisms but also for their practical application to clinical use as bio‐markers. In the last fifty years, lipid peroxidation has been the subject of extensive studies from the viewpoints of mechanisms, dynamics, product analysis, involvement in diseases, inhibition, and biological signaling. Lipid hydroperoxides are formed as the major primary products, however they are substrates for various enzymes and they also undergo various secondary reactions. In this decade, F2‐isoprostanes from arachidonates and neuroprostanes from docosahexanoates have been proposed as bio‐markers. Although these markers are formed by a free radical‐mediated oxidation, the yields from the parent lipids are minimal. Compared to these markers, hydroperoxy octadecadienoates (HPODE) from linoleates and oxysterols from cholesterols are yielded by much simpler mechanisms from more abundant parent lipids in vivo. Recently, the method in which both free and ester forms of hydroperoxides and ketones as well as hydroxides of linoleic acid and cholesterol are measured as total hydroxyoctadecadienoic acid (tHODE) and 7‐hydroxycholesterol (t7‐OHCh), respectively, was proposed. The concentrations of tHODE and t7‐OHCh determined by GC‐MS analysis from physiological samples were much higher than that of 8‐iso‐prostagrandin F_{2α}. In addition to this advantage, hydrogen‐donor activity of antioxidants in vivo could be determined by the isomeric‐ratio of HODE (9‐ and 13‐(Z,E)‐HODE/9‐ and 13‐(E,E)‐HODE).