First-principles calculations are established as a powerful tool of increasing importance for studying the physical and chemical properties of materials. The talk will be devoted to recent results derived from highly accurate ab initio calculations, emphasizing the broad range of physical properties accessible to these methods. I will first introduce a new approach to achieve shallow n-type doping in diamond, which is of crucial importance for device applications. Fundamental magnetic properties of II-VI dilute magnetic semiconductors will be then discussed, in light of the increasing interest in spintronic systems. I will also present structure-related optical properties of spinel oxides used as transparent conductors, unraveling new features of these technologically important complex systems. Finally, recent results on the intriguing structural properties of GaN surfaces will be explained.