Unlike conventional refinery processing, downhole upgrading involves implementing catalytic processes in oil-bearing geologic formations. In this way impurities contained in heavy crude oil can possibly be left in the ground or easily separated during oil production, providing an improved crude oil feed for refineries. Additionally, value or viability can be added to an otherwise uneconomic or remote heavy oil deposit. In order to successfully produce improved quality oil via a downhole upgrading project, several processing steps are anticipated: placement of catalysts into an appropriate downhole location, mobilization of reactants over the catalyst bed, and creation of processing conditions necessary to achieve a reasonable degree of catalytic upgrading. Each of these steps has been proven by past application; their combination into a unified below-ground process remains problematic. Downhole processing differs from surface processing in that brine, high steam partial pressures and low hydrogen partial pressures need to be accommodated in the downhole setting. There are no reports of significant downhole catalytic upgrading of crude oil, although examples of thermal upgrading are noted. However, available technology should be amenable to conducting a successful process. Upgrading of heavy crude oil at anticipated downhole processing conditions has been successfully proven in the laboratory. Recently published literature with immediate pertinence to the problems of downhole catalytic upgrading is reviewed with the goal of stimulating research and providing directions for future investigations.