It is well established that many heterogeneous catalysts encounter substantial changes of their properties if comparing the catalyst under turn-over conditions with those found ex-situ. These changes encompass not only structural but also electronic properties rendering a detailed characterization still challenging. A variety of characterization techniques have been developed in recent years toinvestigatecatalytic systems under operando conditions. In this respect it is important to realize that none of these methods allows to obtain a complete picture which requires on the one hand the combination of different techniques and on the other hand knowledge about available techniques and their potential use and their limitations. [more]