Heavy crude oils and bitumen represent more than 50% of all hydrocarbons available on the planet. These feedstocks have a low amount of distillable material and high level of contaminants that makes their production, transportation, and refining difficult and costly by conventional technologies. Subsurface upgrading of heavy crude oils and bitumen is of interest to the petroleum industry mainly because of the advantages compared to aboveground counterparts. This book presents an "in depth" account and a critical review of the progress of industry and academia in the area of subsurface upgrading of heavy, extra-heavy oils and bitumen, as reported in the patent and open literature.