The influence of stoichiometry and n-type doping on the abundance of native vacancies in GaAs was investigated systematically. The most important method used was positron annihilation spectroscopy (PAS). However, in the beginning of the work it was not possible to identify vacancies in GaAs unambiguously by PAS. Thus it was a first goal to reach such an unambiguous identification. For that purpose, a correlation of PAS with scanning tunneling microscopy was used for the first time. With that methodology Ga vacancy Si donor complexes in highly Si doped GaAs were identified. In the following, a methodology was introduced to identify vacancies in GaAs by PAS alone. A combination of positron lifetime spectroscopy and Doppler coincidence spectroscopy together with a theoretical calculation of the annihilation parameters was used. By applying this method, Ga vacancy Te donor complexes were identified in Te doped GaAs. It was shown by identifying also As vacancy Si donor complexes in annealed GaAs:Si that the methodology can be applied universally to identify vacancies in both sublattices of GaAs. In addition, the method is suitable for the identification of defects in thin layers. This was demonstrated for Ga vacancies in epitaxial GaAs layers grown at low temperatures (~200°C). The compensation of positively charged As antisites could be explained by the acceptor type Ga vacancies. The abundance of vacancies was studied on differently high Te doped GaAs which was annealed under defined As vapor in order to reach a thermodynamically well defined state. In these samples, Ga vacancy te donor complexes were found again. The vacancy concentration increased with increasing As vapor pressure during annealing, i.e. As-rich conditions favor the formation of Ga-vacancies. The vacancy concentration was found to increase with Te doping as well. In addition, an increase of the vacancy concentration was observed with decreasing temperature. Such behavior was predicted as a consequence of the so called Fermi-level-effect. It was experimentally verified for the first time. Using the model of th Fermi-level-effect it was possible to describe our data quantitatively. Following that, the occurrence of acceptor-type vacancies in n-type GaAs is caused by an As-rich stoichiometry and by the attempt to compensate the incorporated donors.