Modern solid‐state physics explains the physical properties of numerous materials, such as simple metals and some semiconductors and insulators. But materials with opendandfelectron shells, where electrons occupy narrow orbitals, have properties that are harder to explain. In transition metals, such as vanadium, iron, and their oxides, for example, electrons experience strong Coulombic repulsion because of their spatial confinement in those orbitals. Such strongly interacting or “correlated” electrons cannot be described as embedded in a static mean field generated by the other electrons. The influence of an electron on the others is simply too pronounced for each to be treated independently.