Advancements in ZnO device applications have fostered much interest in the electrical and optical activities of various defects and impurities in the material. Although it has long been known that Group III dopants, such as Al, make efficient donors, the roles of other impurities, such as H and N, are only recently being elucidated. The same is true of the simplest point defects, such as Zn and O vacancies and interstitials. Theoretical work has been essential in identifying and understanding various defects and impurities. For example, theory has shown that H is always a donor (not amphoteric), that the O vacancy is a deep donor, not shallow, and that the Zn interstitial is a shallow donor, in agreement with electron-irradiation (EI) experiments. Recent irradiation studies show that significant defect annihilations take place, even at low temperatures, thus showing why ZnO is so resistant to radiation effects. To develop applications involving electroluminescence, it will be necessary to identify a reliable acceptor dopant, and N, P, and As have been most thoroughly investigated so far. In fact, p-type samples with resistivities < 1 Omega-cm have been demonstrated, but certain questions remain unanswered. (C) 2003 Elsevier B.V. All rights reserved.