AbstractWe present a first-principles study of heteroepitaxial interfaces between GaN and both cubic as well as wurtzite AlN substrates oriented along main cubic or hexagonal directions and of stacking fault interfaces between cubic and wurtzite GaN. Our calculations show that all studied heterostructures are of type I. Valence band offsets for GaN/AlN are nearly independent of the substrate orientation and of the order of 0.8 eV. The valence and conduction band offsets for a stacking fault interface are predicted to be 40 meV and 175 meV, respectively.