Complex investigations of point defects in ZnO are proposed in the present project. Positron annihilation spectroscopy (PAS) including also variable energy PAS using slow positron beam will be used as a principal technique for defect studies. State-of-art ab-initio theoretical calculations will be employed for interpretation of PAS data. Defects in ZnO single crystals will be compared with those in epitaxial and nanocrystalline ZnO thin films. Defects studies will be combined with electrical (temperature-dependent Hall effect, deep level transient spectroscopy) and optical (photoluminescence, optical transmission) measurements in order to find a link between predominant defect configurations and specific electrical and optical properties of ZnO samples. Moreover, in the present project we intend to investigate interaction of hydrogen and nitrogen with point defects in ZnO and influence hydrogen and nitrogen on electrical and optical properties. A new UHV chamber for on-line sputtering of ZnO films will be constructed and connected to slow positron beam. This novel setup enables to perform variable energy PAS investigations of thin ZnO films in-situ during film deposition. It gives us an exclusive possibility to investigate formation of defects and incorporation of impurities into ZnO lattice during film growth.