A free radical is defined as any species containing one or more unpaired electrons (electrons singly occupying an atomic or molecular orbital), whereas reactive species is the collective term for radicals and some other non-radical derivatives of oxygen, nitrogen, or sulfur that can easily generate free radicals and/or cause oxidative damage (Halliwell 2012). As shown in Table 10.1, reactive oxygen species (ROS) include free radicals such as hydroperoxyl radical (HO2), superoxide anion radical (O2−), hydroxyl radical (HO) and peroxyl radical (ROO; e.g., lipid derived), and other species such as hydrogen peroxide (H2O2), singlet oxygen (1O2), hypochlorous acid (HOCl), and peroxide (ROOR; Lü et al. 2010; Carocho and Ferreira 2013).