Abstract The Eocene Green River Formation has long been believed to contain the world’s largest commercial oil shale deposits, having a recently estimated in situ resource of 2 trillion barrels of oil. Most of this resource lies within the Piceance Creek basin in northwestern Colorado, but additional oil shale intervals also occur within the Uinta basin in Utah and the greater Green River basin in Wyoming. The smaller reported magnitude of resources in Utah and Wyoming reflects thinner stratigraphic intervals but may also be due in part to more conservative assessment approaches (Utah) or to less complete assessment data (Wyoming).