Let us consider a TM compound MnO as an example. The electron energy levels of Mn and O atoms are shown in Figure 1.1. An Mn atom has 3d, 4s, and 4p electrons (seven electrons, where the 4s and 4p levels are shown as continuum in Figure 1.1) as valence electrons and 1s, 2s, 2p, 3s, and 3p electrons (18 electrons) as core electrons. The O atom has four 2p valence electrons and 1s and 2s (4 electrons) as core electrons. If we regard MnO as a pure ionic solid, the Mn 4s and 4p electrons are transferred to the O 2p states to form the Mn2+ ion (with fi ve 3d electrons) and the O2− ion (with six 2p electrons), and MnO is stabilized by the ionic force between Mn2+ and the O2−. Actually, there is a considerable covalency character in valence electrons in MnO, so that the Mn 3d and O 2p states are hybridized with each other. The 18 core electrons in Mn and 4 core electrons in O are almost the same as those in free atoms, so that their features are described by atomic physics. The solid state properties of MnO, that is, electric, magnetic, thermal, and other properties, are determined by VES (i.e. the degrees of the ionicity and covalency, the electron correlation, the existence of the energy gap, and so on). In core level spectroscopy, one of the 18 + 4 core electrons is excited by incident x-rays and the electronic states of Mn 3d and O 2p electrons are studied by taking advantage of the well-known character of the core electron states.