The discovery and conceptualization of ferroptosis as regulated, iron-catalyzed cell death driven by excessive lipid peroxidation triggered re-evaluation of lipid hydroperoxides in connection with health and disease. Free and ester forms of polyunsaturated fatty acids (PUFAs) are oxidized in vivo by multiple oxidizing species to produce lipid hydroperoxides as primary products, some purposely while others unintentionally. The detailed analysis of isomer distribution of lipid hydroperoxides enables us to identify the responsible oxidants. Linoleates, the most abundant PUFA in humans, are oxidized to give multiple isomers of hydroperoxyoctadecadienoates (H(p)ODEs) as primary major products, racemic trans, trans-9- and 13-H(p)ODEs, 13(S)-cis, trans-H(p)ODE, and 10- and 12-H(p)ODEs being specific biomarker for the oxidation by free radicals, lipoxygenase (LOX), and singlet oxygen, respectively. Cholesterol is another important lipid and its hydroperoxides are produced solely by non-enzymatic oxidation, the major products being cholesterol 7-hydroperoxide and 5-hydroperoxide by free radicals and singlet oxygen, respectively. The available data obtained from human samples show that lipid hydroperoxides are produced in vivo primarily by free radical mediated lipid peroxidation and that the contribution of LOXs and singlet oxygen is small. Multiple antioxidants having different functions play their respective roles in the physiological defense network against detrimental lipid peroxidation and ferroptosis. The fact that lipid hydroperoxides are produced in vivo mainly by free radical mediated lipid peroxidation suggests that radical scavenging antioxidants act as essential ferroptosis inhibitors, which was substantiated by many studies. Considering the reactivity and physiological concentrations, it may be said that vitamins E and C play the primary roles as biological radical scavenging antioxidants against ferroptosis by synergistic interactions. Novel synthetic antioxidants with higher reactivity than natural antioxidants have been reported and their biological effects should be assessed. The factors that determine antioxidant effects in vivo are critically reviewed.