This book presents recent developments in high-resolution X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) in terms of time-resolved and surface enhancement essential to understanding catalytic reactivity and capitalizing on core-level X-ray spectroscopy at fourth-generation light sources (XFELs). The text describes all spectrometers developed to perform core-level X-ray spectroscopy, highlighting the advantages and disadvantages of each. It provides detailed descriptions of methods to elucidate aspects of catalysts under working conditions, such as active sites and molecule adsorption. It also considers future opportunities in XFELs.-- Provided by publisher.