Free radicals have been implicated and considered as associated risk factors for a variety of human disorders including neurodegenerative diseases, although it is not yet clear whether oxidative stress acts as a causative agent of neuronal degeneration. In human tissues, a condition of oxidative stress can be revealed through searching for specific biomarkers of oxidative damage to lipids, proteins and nucleic acids. Markers of oxidative damage to lipids include 4-hydroxynonenal, malondialdehyde, lipid hydroperoxides and isoprostanes, and thiobarbituric acid–reactive substances. Protein carbonyls and protein nitration are common markers of oxidative damage to proteins. The levels of 8-hydroxyguanine, or alternatively 8-hydroxy-2’- deoxyguanosine, can be measured in brain specimens, blood and urines, and are commonly used as a marker of oxidative DNA damage. Besides DNA a growing body of evidence indicates that also RNA undergoes oxidative damage. Studies have been performed also on markers of antioxidant defence such as oxidative modifications of plasma proteins, the activity of enzymes of the antioxidant defence (superoxide dismutase and catalase), the levels and the activity of proteins involved in the repair of oxidative DNA damage and the state of components of blood glutathione system. Significant biological changes related to a condition of oxidative stress have been found not only in brain tissues but also in biological fluids such as urine, peripheral blood or cerebrospinal fluid, and in peripheral tissues such as blood cells and fibroblasts of individuals affected by Alzheimer’s disease, Mild Cognitive Impairment, Parkinson's disease and Amyotrophic Lateral Sclerosis.