The study of catalysts under in-situ conditions enables the catalyst scientist to identify and understand the important steps, e.g., the formation of important reaction intermediates and active sites, and stages in a catalyst?s lifetime, such as activation/deactivation. The development of characterization methods as well as the design and construction of appropriate in-situ cells and reactor probes are inevitable in order to obtain such insight. Both spectroscopic and scattering techniques have been used in attempt to understand quantitative structure/composition-activity/selectivity relationships in catalysis. Armed with such detailed knowledge about the catalyst, it is then possible for scientists to design, in a more rational way, new and efficient catalysts for sustainable production of bulk and fine chemicals as well as for the removal of harmful compounds in industrial catalytic processes. This chapter provides an overview of the in-situ characterization techniques available for investigation of catalytic materials. The possibilities and limitations of the methods are discussed and illustrated with numerous case studies.