Electron paramagnetic resonance (EPR) spectroscopy is a method for characterizi ng structure, dynamics and spatial distribut ion of paramagneti c ions. Diamagnetic materials can be studied by using spin probes which are stable paramagnetic species such as nitroxide radicals and transition metal ions. Due to the presence of at least one unpaired electron many paramag netic species are chemically active. Therefore EPR spectroscopy is a valuable technique for obtaining detailed information on the geometric and electroni c structure of various materials. 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.