Kohn-Sham density-functional theory is a very successful quantum-mechanical method for describing the electronic structure of materials. It is a computationally efficient method that, with modern supercomputers, is able to routinely simulate up to 1000 atoms. It serves as the fundamental tool that our research group uses to investigate material properties and processes at atomistic length-scales. It is ab initio in the sense that once the atom positions and chemical identities are defined, the total energy is found by solving the Kohn-Sham equations.