This volume records the proceedings of a Forum on The Fundamentals of Electron Density, Density Matrix and Density Functional Theory in Atoms, Molecules and the Solid State held at the Coseners' House, Abingdon-on-Thames, Oxon. over the period 31st May - 2nd June, 2002. The forum consisted of 26 oral and poster presentations followed by a discussion structure around questions and comments submitted by the participants (and others who had expressed an interest) in advance of the meeting. Quantum mechanics provides a theoretical foundation for our under­ standing of the structure and properties of atoms, molecules and the solid state in terms their component particles, electrons and nuclei. (Rel­ ativistic quantum mechanics is required for molecular systems contain­ ing heavy atoms.) However, the solution of the equations of quantum mechanics yields a function, a wave function, which depends on the co­ ordinates, both space and spin, of all of the particles in the system. This functions contains much more information than is required to yield the energy or other property.