As discussed earlier, Thomas and Fermi were the first to contemplate a model for the electron many-body problem based uniquely on the electron density . Due to the severe shortcomings of this method, they probably never imagined that an exact theory could be based on the density. However, almost forty years later, Hohenberg and Kohn proved in a seminal paper  that this was indeed possible. In two remarkably powerful theorems they formally established the electron density as the central quantity describing electron interactions, and so devised the formally exact groundstate method known as density functional theory (DFT). The two Hohenberg-Kohn theorems are now described.