Probing events that occur in a catalyst under working conditions is a very important topic in many fields of chemistry. In order to identify reaction intermediates and active sites in a working catalyst, the development of characterization techniques, the design and construction of appropriate in-situ cells and reactor probes are inevitable. Various types of spectroscopy, diffraction methods and scattering techniques can be used to achieve the ultimate goal of determining and understanding quantitative structure/composition-activity/selectivity relationships in catalysts. Therefore, such detailed knowledge about the active sites should enable scientists to design - in a rational way - new and efficient catalysts for sustainable production of bulk and fine chemicals as well as for the removal of harmful compounds in industrial catalytic processes. This book aims to give an overview of the different characterization techniques currently available for performing in-situ studies on catalytic materials. Both the possibilities and limitations are illustrated by many case studies. This is a unique book providing a comprehensive coverage of the latest developments in this very important and rapidly expanding field of catalysis.