Electron paramagnetic resonance (EPR) is a technique that allows the detection of free radicals formed in chemical or biological systems. In addition, it studies the symmetry and electronic distribution of paramagnetic ions. This is a highly specific technique because the magnetic parameters are characteristic of each ion or free radical. The physical principles of this technique are analogous to those of NMR, but in the case of EPR, electronic spins are excited instead of nuclear, that is interesting in certain electrode reactions.