The massive size of the oil shale resource in the Western USA, particularly in the Green River Basin, has attracted numerous commercialization attempts from industry over the last 100 years. Although great sums of money have been invested and many professional careers have been devoted to the challenge, efforts thus far have not resulted in a commercial oil shale industry. For more than 40 years, Shell has been active in the Green River oil shale seeking a process that could develop the 800 billion bbls of oil resource (RAND Corporation). Since 1980 Shell has focused on the In-situ Conversion Process (ICP) in which oil shale is heated by thermal conduction from a closely spaced array of electric resistance heaters. At approximately 650°F, the kerogen present in the oil shale is converted to oil, gas and water that can be produced by conventional means. Although the process is subject to mining statutes and regulations in the State of Colorado, no traditional mining is required. This paper provides a brief history of attempts to commercialize oil shale in the United States and an overview of Shell's ICP technology, including its 1940s Swedish oil shale roots. It focuses on Shell's seven field pilots conducted in Colorado that address ICP recovery, heater testing, and freeze wall construction and performance.