Environmental concerns stemming from the use of conventional solvents and from hazardous waste generation have propelled research efforts aimed at developing benign chemical processing techniques that either eliminate or significantly mitigate pollution at the source. This paper provides an overview of heterogeneous and homogeneous catalysis in dense phase CO2, considered a green solvent. In addition to solvent replacement, the demonstrated advantages of using dense phase CO2 include the enhanced miscibility of reactants, such as O-2 and H-2 which eliminate interphase transport limitations, and the chemical inertness Of CO2. Further, the physicochemical properties Of CO2-based reaction media can be pressure-tuned to obtain unique fluid properties (e.g. gas-like transport properties, liquid-like solvent power and heat capacities). The advantages Of CO2-based reaction media for optimizing catalyst activity and product selectivity are highlighted for a variety of reactions including alkylation on solid-acid catalysts, hydrogenation on supported noble metal catalysts and a broad range of homogeneous oxidations with transition metal catalysts and dioxygen as an oxidant. Through these examples, the need is emphasized for a systematic approach to research and development of supercritical carbon dioxide based processes, taking into account conventional multi-phase reaction engineering principles, catalytic chemistry and phase behavior.