Reactive oxygen species or peroxynitrite, coming from the reaction of nitric oxide and superoxide anion, are strong oxidants capable of damaging lipids, proteins and DNA. The oxidative products issuing from each biomolecule are complex and multiple. Reactivity, the mechanism of production and the products formed vary depending on the free radical (superoxide anion, hydroxyl radical, peroxynitrite) and the molecular target (phospholipids, cholesterol, aromatic or aliphatic amino acids, puric or pyrimidic bases). Some of these oxidative products are markers of oxidative stress. For example malondialdehyde and isoprostanes are oxidative markers of lipids, carbonylated proteins of proteins and 8-oxo-guanine or 8-oxodeoxyguanosine of DNA. However other products are also produced, as is the case of the reaction of peroxynitrite with tyrosine which leads to the formation of 3-nitrotyrosine. The quantification of 3-nitrotyrosine is labor-intensive and requires specific equipment. The major problem when searching for the most appropriate marker for a given disease is the great diversity of oxidative products formed depending on the nature of the free radical involved.