First principles methods, based on either Hartree-Fock approximation or density functional theory, are now routinely used to gain an understanding of structural, electronic and optical properties of a wide variety of materials. In 1980s, the amount of computer power available had restricted such methods to idealized problems. Recently, however, the role of modeling based on first principles methods has expanded to include treatment of specific and realistic materials problems due to enormous increase in scientific computational capacity. In this talk, a brief review of first principles methods together with the calculated results on properties of the bulk, surface and nanoclusters of gallium nitride (GaN) will be given. As a further demonstration of the predictive capabilities of these methods, electronic properties of hydro-silsesquioxanes and germanium-tin ordered alloy will also be presented.