Oil-shale deposits are found in many parts of the world. The), range in age front Cambrian to Tertiary and were formed in a variety of marine, continental, and lacustine depositional environments. The largest known deposit is the Green River oil shale in western United States. It contains an estimated 215 billion tons of in-place shale oil (1.5 trillion U.S. barrels). Total resources of a selected group of oil-shale deposits in 33 countries is estimated at 411 billion tons of in-place shale oil which is equivalent to 2.9 trillion U.S. barrels of shale oil. This figure is very conservative because several deposits mentioned herein have not been explored sufficiently to make accurate estimates and other deposits were not included in this survey.