Publisher Summary This chapter reviews the latest calculations of the electronic properties of intrinsic and heavily doped 3C-, n H-SiC (n = 2,4,6) and zb-, wz-III-N (III = B, Al, Ga, In). It presents the first-principles, fully relativistic full-potential linearized augmented plane wave (FPLAPW) calculation of the electronic band structure, the effective electron and hole masses, and the dielectric constants for intrinsic materials masses that is vital for the understanding of the electronic transport properties in these semiconductors. It focuses on the non-parabolicities of the energy bands near the band edges. The local density approximation (LDA) bandgap error can be corrected by a quasi-particle energy shift that has an effect on the high-frequency dielectric constant. This chapter calculates the static dielectric constant assuming constant optical phonon frequency dispersion. The anisotropy of the dielectric function is small in wz SiC and III-N. The effective electron and hole masses both as the bare effective masses and by including the polaron interaction has been presented in the chapter. It is crucial to take into account the spin-orbit interaction for determining the effective hole masses even in light semiconductor materials.