Interactions between electrons determine the structure and properties of matter from molecules to solids.To describe interacting electrons,the simple three-dimensional electron density can be used as the basic variable within density functional theory,negating the need in many cases for the massively complex many-dimensional wave function.This lecture will review the concept of electron density and many of its remarkable features,including latest developments.Electron density determines the energetics in electron transport,solar energy harvesting and conversion,drug design in medicine,catalytic processes in enzymes,and many other processes in science and technology.Particularly,chemical and redox reactions in biological systems will be featured.Electron density can be used to visualize also non-covalent interactions such as hydrogen bonds,van der Waals attractions and steric repulsions.Electron density also naturally leads to the concepts of fractional charges and fractional spins,which are key to understanding and progress.