X−ray photoemission spectroscopy (XPS) is a ’’semisurface’’ technique that can be modified to study bulk properties, surfaces, or adsorbed species. As a bulk probe it is capable of yielding bandstructure information. It can distinguish amorphous from crystalline materials and is sensitive to magnetic and spatial order. Sensitivity to surface character is less pronounced. Relaxation effects are prominent in XPS spectra. Core−level binding energies of adsorbed species yield structural information if relaxation is considered. The same should be true for valence orbitals. Heterogeneous catalysis can be understood in a general sense as arising from the same origins as the relaxation energies in photoemission.