A number of homogeneous and heterogeneous catalytic reactions have been successfully performed in supercritical fluids (SCFs). An overview on recent developments in the areas of alkylation, isomerization, hydrogenation, partial oxidation, amination, and CO2-fixation using heterogeneous catalysts and supercritical fluids is given. Additionally, strategies towards a more fundamental understanding of catalytic reactions in supercritical fluids are outlined. One aspect is the identification of phase behavior in such multicomponent systems. Their complexity and the input of in situ monitoring is discussed. It is proposed that binary fluid mixtures are an ideal guide for simplifying and understanding the phase behavior in reaction mixtures. In order to strengthen the future use of this knowledge, e.g., for optimization of reactions in SCFs, an overview on the different topologies of binary mixtures is given. Another aspect is in situ characterization of the catalytic reaction and their intermediates, the intermolecular interactions in the fluid, the heterogeneous catalyst phase, and the solid/fluid interphase. The opportunities of various available spectroscopic tools, applicable in situ, are also reviewed by referring to examples from homogeneous catalysis or low-pressure studies.