This paper offers a review of estimates of ultimately recoverable resources (URR) of non-renewable energy sources: coal, conventional and unconventional oil, conventional and unconventional gas, and uranium for nuclear fission. There is a large range in the estimates of many of the energy sources, even those that have been utilized for a long time and, as such, should be well understood. If it is assumed that the estimates for each resource are normally distributed, then the total value of ultimately recoverable fossil and fissile energy resources is 70,592 EJ. If, on the other hand, the best fitting distribution from each of the resource estimate populations is used, a the total value is 50,702 EJ, a factor of around 30% smaller. (C) 2012 Elsevier Ltd. All rights reserved.