The potential a valence electron feels in a crystal is not a predetermined one. The electrons interact with each other by Coulomb forces and the one-electron theory is to be understood at most in the sense of a self-consistent approximation. Diagrammatic perturbation theory may lead to higher many-body corrections. In a semiconductor an electron in the conduction band may move “freely,” while in the valence band the same is true for the holes (absent electrons). This leads to the picture of an electron–hole plasma. A single electron–hole pair by Coulomb attraction may be bound to an exciton with hydrogen-like spectrum.