In the last decades, density functional theory (DFT) has become one of the most used simulation methods to describe the quantum mechanical behaviour of electrons in an electrostatic potential of ion cores and other electrons. Such a system is known to us as a material or matter, in general. With DFT and close relatives, various ground state and excited state properties of the electrons on the nanoscale can be computed, for example the atomic configuration of a molecule on a surface. This lecture offers a pedagogical presentation of the conceptual ideas underlying DFT and its close relatives (as Hartree-Fock, wavefunction correlation methods and GW). With these methods, it is possible to treat comparably large systems (up to thousands of atoms) enabling direct comparison to experiments in many areas. Applications to problems in nanoscience, solid-state physics and quantum chemistry will be discussed. We will also discuss the extension of DFT to time-dependent problems, for example electron dynamics triggered by ultrashort laser pulses. In exercises, students apply state-of-the-art DFT packages to scientifically relevant examples.