Materials derived from copper oxide, Cu2O, are leading candidates for p-type transparent conducting oxides (TCO). We use first principles density functional theory (DFT) to investigate the defect chemistry of Cu2O, CuAlO2 and SrCu2O2. The origin of the p-type character of these materials is the facile formation of acceptor defects, in particular copper vacancies. Defect formation can be tuned so that under oxidising conditions formation of Cu vacancies is promoted. Formation of compensating defects is unfavourable. The defect chemistry of these TCO materials is remarkably similar and the present results demonstrate that acceptor defects will dominate the defect chemistry of Cu2O materials. (C) 2008 Elsevier B.V. All rights reserved.