Valence band electrons in solid are the charge carriers and from the chemical bonds. Thus the electronic states of the valence band electrons are the valence and conduction bands. Each electronic state in such a band is characertized by a binding energy and a momentum. The relation between these two units and the number of occupied (und thus also unoccupied) states determine the chemical reactivity and the electric properties. With ARPES and IPES/BIS the electronic band structure of the occupied and unoccupied states can directly be determined.