The book provides an in-depth insight into the fundamental aspects of electronic structure of solids through powerful technique of photoelectron spectroscopy. The exhaustive analysis of the core-level binding energy provided detailed information of the interactions happening inside a solid. The binding energy of a core-level is characteristic of the element in the compound, where shift of the core-level determined the valence state of the constituent elements. The area under the core-level presented a quantitative estimate of the composition. The variation of core-level intensity and line shape with adlayer deposition furnished information of the adlayer growth mode and possible interface reactions. The nature of collective excitation is established through thorough analysis of bulk and surface Plasmon line shape. The unique use of photoelectron spectroscopy is illustrated by bubble radius estimation using core-hole screening and relaxation effects. Although there are books on photoelectron spectroscopy, they lack in unfolding and conveying the beauty of using core-levels to understand various fundamental facets of electronic structure.