This book covers recent developments in X-ray light sources, detectors, crystal spectrometers, and photon-in-photon-out core level spectroscopy techniques. Providing a comprehensive overview of this exciting field, the text addresses photon-in-photon-out core level spectroscopy applications for the study of catalytic systems, highlighting hard X-ray measurements. Complete with guidelines for the use of this type of spectroscopy, it describes the advantages and disadvantages of each spectrometer developed to perform core level X-ray spectroscopy, detailing methods to elucidate aspects of catalysts under working conditions, such as active sites and molecules adsorption.