Impurities play a noticeable role in a system’s electrical properties, such as the conductivity, and thus are of interest in solid state physics. Random alloys in which at least two atomic species take random positions in a crystal’s lattice sites fall within the realm of disordered systems. The vicinity around the substitutional or the interstitial impurities is accompanied by a lattice distortion. This chapter studies a simple theory that can be applied in order to obtain the energy level associated with a substitutional isoelectronic impurity in a crystal system. Impurity solutions inside the band are referred to as resonances, and solutions outside the band are generally known as deep levels, depending on how far they lie from the band. The perturbation affects the density of states, and the impurity contribution to it is shown by a large peak at the impurity bound state.