We have investigated the electronic structures of bulk GaSe and GaTe as well as the nature of defect states
associated with substitutional impurities and vacancies in GaSe and GaTe. These calculations were done using <i>ab initio</i>
density functional theory and supercell models. We find that the Ga-Ga dimers play an important role in the formation of
defect states. Analysis of the charge densities and the band structures associated with the defect states indicates that they
are strongly localized. Theoretical results are in good agreement with experiment for Cd<sub>Ga</sub> and V<sub>Ga</sub> in GaSe and for V<sub>Ga</sub>
in GaTe. The effect of spin-orbit interaction on the band structure of GaTe has been investigated; it is found that the top
valence bands at the Γ-point shift up in energy by ~ 0.1 eV due to the mixing of Te p<sub>x</sub>-p<sub>y</sub> and p<sub>z</sub> bands.