Free radicals are species that have an independent existence, for however brief a period, and contain at least one unpaired electron. The production of free radicals is inherent to normal cell behavior. Many redox processes produce and consume them. Superoxide (O •- 2 ) is the most typical oxygen-containing species enclosing a free radical. It is formed as a side product of the energy production process in the mitochondrion. O •- 2 , peroxides and hydroxyl radicals (OH • ) are included under the general heading of reactive oxygen species (ROS). The unbalance of free radicals leads to the so-called oxidative stress, and it is at the origin of multiple diseases like cancer, diabetes, Alzheimer's, or Parkinson's diseases. The human brain is a highly aerobic organ with high oxygen consumption, with a high energy requirement of neurons driven by mitochondrial oxidative phosphorylation. Part of this oxygen is converted into ROS, which in a healthy individual are effectively detoxified by several antioxidants, like enzymes (glutathione peroxidase and catalase for hydrogen peroxide, 51 and superoxide dismutase for superoxide ) or low molecular mass antioxidants. These antioxidants can perform a radical-scavenging process with ROS, receiving/donating an electron from/to a radical, forming stable byproducts. For instance, in lipid peroxidation within the mitochondrial membranes, the reduced form of the coenzyme Q 10 (ubiquinol, UQH 2 ) acts as the main chain-breaking antioxidant that decreases the damage of this process. The active center of the Q 10 antioxidant is formed precisely by hydroquinone (QH 2 ).