As density functional theory (DFT) has expanded in the academic and industrial research communities, ready-to-use, commercial, and free software has made the task of carrying out calculations much easier. Unfortunately, this has caused this important standard tool in quantum chemistry and solid-state physics to be treated as a mysterious black box by many users. In a Tutorial Review in the International Journal of Quantum Chemistry, targeted at senior undergraduates, first-year graduate students, and scientists new to the field, Kieron Burke and graduate student Lucas Wagner focus on the basic machinery operating at the heart of this powerful quantum mechanical modeling method. The cover image highlights the electronic density, rather than the wavefunction, as the key variable in DFT.