Peroxides are chemical compounds in which two oxygen atoms are linked together by a single covalent bond. Hydrogen peroxide (H2O2), the simplest peroxide, is formed through different routes in biological systems: (1) superoxide radical (O2 ) dismutation, which can be spontaneous, or, depending on the organism and cellular compartment, catalyzed by different superoxide dismutases (SODs) [1–3]; (2) one-electron O2 reduction, such as during aconitase oxidation [4,5]; or (3) direct two-electron reduction of oxygen, which can be catalyzed by the divalent 50activity of several oxidases, including xanthine oxidase, Ero1, aldehyde oxidase, and monoamine oxidase [6–10] (Figure 3.1). In turn, peroxynitrous acid (ONOOH)* is a peroxy acid (or peracid), that is, a compound that contains an acidic –OOH group. ONOOH is the conjugated acid of peroxynitrite anion (ONOO) (pK a = 6.6–6.8 [11–13]), whose main biological source is the rapid recombination reaction between O2 and nitric oxide (NO) radicals [14–16]† (Figure 3.1). NO is a small and lipophilic radical that can diffuse through membranes [20,21]. On the contrary, the charged nature of O2 (the pK a of the conjugated acid hydroperoxyl radical (HO2 ) is 4.8 [22]) limits its diffusion through membranes to those expressing anion channels, or to those delimiting compartments with acidic pH that allow O2 protonation at a significant proportion [23]. Moreover, the diffusion distance for O2 is estimated to be very short (~ 0.5 μm) [24]. Thus, ONOOH as well as H2O2 arising from O2 are expected to be formed primary at the main sites of O2 generation. Depending on the cell type, those sites could be mitochondria, phagosomes of inflammatory cells, as well as the extracellular space. Additionally, O2 can be formed through redox-cycling of xenobiotics at different cell compartments [25].