Developing catalysts by design instead by trial and error – This is a major goal in catalysis which requires detailed knowledge about the specific sites involved in reactant adsorption, electron transfer and formation of intermediates and products. Usually, catalysts adopt their active state only during reaction and this may not be preserved at the end. For exploring what catalysts are doing while they are working we must directly look inside by specifically tailored operando spectroscopy. The last two decades have seen impressive improvements of such spectroscopic techniques from model conditions towards real reaction conditions and higher spatial and time resolution. Nevertheless, obtaining all experimental information needed to set up verified reaction mechanisms beyond assumptions and guesses is still a big challenge since it requires the use of multiple spectroscopies without compromising realistic reaction conditions.