The performance of silicon based microelectronic circuits reaches the end of the roadmap. New material systems are required for further improvements in speed and power consumption. Germanium is a possible candidate to substitute silicon for microelectronic devices. Its hole mobility is the highest of all semiconductor materials. Together with its lower band gap it could be an ideal material for energy-saving devices. This thesis is dedicated to first principles studies of the Ge/GeO2 interface through hybrid density functional theory. The substoichiometric region of the interface is of special interest. A wide substoichiometric region is supported by total energy calculations of a set of crystalline model systems. An unexpected structure organization was found through molecular dynamics simulations of substoichiometric GeO. We found that a majority of germanium and oxygen atoms are threefold coordinated, forming valence alternation pairs (VAPs). A detailed energetic analysis located the VAPs in the low-oxygen region of the interface. VAPs show interesting properties : They are prone to charge trapping. The electron trapping level might explain the bad performance of n-type doped devices. Furthermore, VAPs might be at the origin of the difficulties of H passivation at the Ge/GeO2 interface. Since threefold Ge atoms are negatively charged and threefold O atoms are positively charged, VAPs give rise to dipoles. These dipoles may reduce the interface dipole created by the electronegativity difference in the Ge-3O bond. With this mechanism, we can explain the wide range of experimental valence band offsets (VBOs) with the occurrence of different density levels of VAPs at the Ge/GeO2 interface. This suggestion is further confirmed by the determination of the VBO and Ge 3d core-level shift for an atomistic model structure of the Ge/GeO2 interface. Both values are systematically lower than typical experimental values for the Ge/GeO2 interface. Taking the extra dipole into account, our calculated VBOs and XPS shifts are in excellent agreement with experimental values. These results confirm that the structural properties of the Ge/GeO2 interface deviate significantly from its Si counterpart.