On the other hand, paramagnetic defects in crystalline or amorpho us solid materials such as semiconduc tors, optical glasses and ceramics can be stable for very long times. Such defect or impurity centers may significantly influence electrical and optical properties of these materials even at very low concentrations . These defects can also be used as special kind of spin probe. The advantage of EPR spectroscopy is its extreme sensitivity to the microscopic environment of the paramagnetic center. Therefore this technique is applied to study the nature of doped Co 2+ ions in the ZnO lattice.