High-Severity Fluid catalytic cracking (HS-FCC) is a new process for the conversion of heavy oils into lighter hydrocarbon products and petrochemical feedstocks. Research teams from Japan and Saudi Arabia are jointly developing this technology. The HS-FCC process combines mechanical modifications to conventional FCC with changes in process variables and catalyst formulations. The process consists of two interconnected gas-solid fluidized bed reactors: the downer reactor and the regenerator reactor. The main operating regime of the process is a special down-flow reactor system, high reaction temperature, short contact time, and high catalyst/oil ratio. The process has been successfully tested in a 30 bbl/day demonstration plant at a site in Saudi Aramco's Ras Tanura refinery. As with most reactor designs involving competing reactions and secondary product degradation, there is a concern over catalyst-feed contacting, back-mixing, and control of the reaction time and temperature. The paper highlights the results obtained from the demonstration plant and summarizes the efforts to further develop and commercialize the HS-FCC technology.