The results of ab initio calculations for vacancies and divacancies in GaN were presented. Particular attention was paid to N vacancies and mixed Ga-N divacancies in negatively charged states which, in n-type GaN, were found to be energetically comparable with Ga vacancies. It was also demonstrated that the activation energy for self-diffusion over the N sub-lattice was lower than over the Ga one for all Fermi-level positions. This implied that N vacancies were major defects in samples annealed at high temperatures. The direct observation of N vacancies by positron annihilation was considered.Nitrogen Vacancies as Major Point Defects in Gallium Nitride. M.G.Ganchenkova, R.M.Nieminen: Physical Review Letters, 2006, 96[19], 196402 (3pp)