This book is devoted to the emerging field of techniques for visualizing atomic-scale properties of active catalysts under actual working conditions, i.e. high gas pressures and high temperatures. It explains how to understand these observations in terms of the surface structures and dynamics and their detailed interplay with the gas phase. This provides an important new link between fundamental surface physics and chemistry, and applied catalysis. The book explains the motivation and the necessity of operando studies, and positions these with respect to the more traditional low-pressure investigations on the one hand and the reality of industrial catalysis on the other. The last decade has witnessed a rapid development of new experimental and theoretical tools for operando studies of heterogeneous catalysis. The book has a strong emphasis on the new techniques and illustrates how the challenges introduced by the harsh, operando conditions are faced for each of these new tools. Therefore, one can also read this book as a collection of recipes for the development of operando instruments. At present, the number of scientific results obtained under operando conditions is still limited and mostly focused on a simple test reaction, the catalytic oxidation of CO. This reaction thus forms a natural binding element between the chapters, linking the demonstrations of new techniques, and also connecting the theoretical and experimental studies. Some first results on other reactions are also presented. If there is one thing that can be concluded already in this early stage, it is that the catalytic conditions themselves can have dramatic effects on the structure and composition of the surfaces of catalysts, which, in turn can greatly affect the mechanisms, the activity, and the selectivity of the chemical reactions that they catalyze. .