In this letter, we compare the properties of bulk and epitaxial ZnO. The ZnO thin films were grown on GaN templates and on ZnO single crystals by vapor phase deposition using Zn and NO2 as precursors. We use high-resolution x-ray diffraction to resolve the structural properties. The rocking curves of the bulk crystal are extremely broad caused by a mosaic structure of the substrate. The homoepitaxial ZnO film mimics the properties of the ZnO substrate whereas ZnO films on GaN templates showed superior rocking curve half width as small as 230 arcsec. The optical properties are investigated by temperature-dependent photoluminescence. Different donor and acceptor bound excitons can be distinguished for a half width of the recombination lines less than 1 meV. Free exciton emission is already detectable at liquid-He temperatures proofing the high quality of the epitaxial films. © 2002 American Institute of Physics.