In this chapter surface-sensitive X-ray diffraction is introduced as an important crystallographic tool for the investigation of surfaces and nanostructures under high pressure reaction conditions and elevated temperatures which are relevant for industrial catalysis. After the introduction surface-sensitive X-ray diffraction methods are briefly explained and specialized instrumentation developed for the in situ investigation of surfaces and nanostructures across the pressure gap is presented combined with simultaneous measurement of the concentrations of reactants and products. In the following an overview of the experimental results is given: First the (near)-ambient pressure oxidation of 3d, 4d, and 5d transition metals is discussed which are relevant for oxidation catalysis. Afterwards catalytic reaction experiments in batch mode are reported, followed by an overview of current research using a flow reactor for surface-sensitive X-ray diffraction. Finally a perspective is given for future research directions.