MgO is a very interesting material and it has been extensively analysed both; experimentally and theoretically. The electronic structure of the MgO crystal has been calculated with the perturbed – ion (PI) method. The PI atomic like orbitals are eigen functions of Fock operators that contain nuclear coulombic and exchange lattice potentials plus lattice projection operators enforcing the ion-lattice orthogonality. These lattice-consistent ionic orbitals from crystalline basis set that may be useful in a number of applications. The PI bonding picture of MgO consists of lattice stabilized Mg2+ and O2– ions described with well separated wave functions. The PI electron density of Mg2+ is very close to the freeion function, but that of the oxide is more concentrated than the density of O2– in vacuo. The PI densities are tested and compared with others by computing diamagnetic susceptibilities, form factor and the change of electronic kinetic energy upon the crystal formation. Also it shows important applications as a catalyst and it is frequently used as a host lattice for transition-metal impurities in a large variety of optical and magnetic studies.