Most SCEs of practical interest are not intrinsic semiconductors but they are often doped with foreign atoms to improve their conductivity or other physical and chemical properties. The introduction of dopants has a direct impact on the SCEs' electronic structure as new electronic states are created. The SCE electronic structure can be also be modified by the presence of polaronic states, i.e. localized electronic states where either an electron or hole becomes spontaneously trapped on a single or few atoms due to lattice distortions. Finally, near an SCE surface, specific surface states may appear due to breaking of the 3D periodicity. When the concentration of surface states is high and when their energy levels fall in the band gap, the Fermi level may remain efficiently fixed at these energies. As a result, the surface becomes charged and most of the electrostatic potential drop takes place on the electrolyte side while the drop within this SCR remains rather unchanged as the electrode potential is applied. This phenomenon is known as Fermi-pinning, which means that the Fermi level, the inner potential, and the electrode potential remain essentially fixed while the surface charge can be drastically changed. Because the Fermi level at equilibrium is aligned with the redox potential of the electrolyte species, Fermi-pinning has a crucial impact on the reactivity of SCEs.