X-ray photoelectron spectroscopy (XPS) studies the chemical properties of solids through the photoelectric effect. In a XPS experiment a solid is irradiated with light of few tens to several hundred eV. The core-level electrons emitted from the sample carry element-specific information. By analyzing the energy spectrum of these electrons it is possible to detect the presence of atomic species, their chemical state and local bonding, their relative abundance (stoichiometry) and spatial distribution. The light sources used in XPS experiments range from X-ray tubes to synchrotrons that are optimized for different parts of the electromagnetic spectrum.