X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy (KE) and number of electrons that escape from the top 1 to 10 nm of the material being analyzed. This book reviews research in the field of X-ray photoelectron spectroscopy including: XPS studies from industrial and bioactive glass to biomaterials and applied to the atomic structure analysis of silicate glasses thin layers; XPS as a tool in the study of nanostructured titanium and in commercial PET surfaces in biotechnological applications and others.