We have used positron annihilation spectroscopy to study vacancy defects in ZnO single crystals grown by various methods from both commercial and academic sources. The combination of positron lifetime and Doppler broadening techniques with theoretical calculations provides the means to deduce both the identities and the concentrations of the vacancies. The annihilation characteristics of the Zn and O vacancies have been determined by studying electronirradiated ZnO grown by the seeded vapor phase technique. The different ZnO samples were grown with the following techniques: the hydrothermal growth method, the seeded vapor phase technique, growth from melt (skull melting technique), and both conventional and contactless chemical vapor transport. We present a comparison of the vacancy defects and their concentrations in these materials.