This chapter discusses first complementarities between Raman and X-ray diffraction (XRD), and also including X-ray absorption spectroscopy (XAS). This unique combination has proven its great value in identifying the active sites and species and in understanding factors that influence the long-term performance of catalysts, and several examples are reviewed. The combination of these techniques can be exploited with an “in-situ but separated multitechnique approach” or with an “in-situ and simultaneous multitechnique approach”. The chapter gives an introduction to the relatively old concept of modulation enhancement (ME) that is only recently applied in catalysis. The more specific term of modulation excitation spectroscopy is generally used instead of the generic term of ME. Both terms cover the idea of increasing the sensitivity and introducing selectivity into the measurements. Finally, the chapter points out that no matter how powerful a technique will be, it will only provide answers on how reactions take place.