The results of extensive theoretical studies of the properties of interfaces and surfaces of the wide-gap III-V nitride semiconductors are reviewed. The phenomena that we have investigated include band offsets and transport properties in nitride-based devices, surface reconstruction energetics and adsorbate-substrate interactions. In the modelling of realistic blue-laser multiquantum well structures such as In0.2Ga0.8N/In1−x
N (x > 0.8) superlattices, pyro- and piezo-electric effects are found to be strong enough to reduce the interband recombination rate and limit the efficiency of an actual device. We have also studied the influence of growth conditions and surface polarity upon the morphology of (0001) GaN surfaces, which are the primary growth faces. The charge transfer between the Ga and N atoms in GaN and the very large electronegativity of nitrogen are found to play decisive roles in determining the stable reconstructions. The behavior of Mg at the Ga-terminated (0001) surface has also been investigated. We find that Mg in the Gasubstitutional site is preferred to adatom sites on the surface. Further, our results suggest that Mg prefers to segregate to the surface.