The study of solids and their electronic band structure by photoelectron ejection received an impetus with the original development of molecular PE spectroscopy, and has undergone a second expansion with the application of synchrotron radiation sources. From the physico-chemical point of view, bonding and behaviour of adsorbates are of the greatest interest, particularly because of their relevance to heterogeneous catalysis. The study of these topics is a multi-disciplinary field in which no single technique is preeminent. Many techniques are now available such as, X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron spectroscopy (UPS), Auger Electron Spectroscopy (AES), etc.