Oil and gas resources in each of the 24 plays within the National Petroleum Reserve in Alaska (NPRA) were estimated using a play analysis. Assessors specified geologic attributes, risks, and number of prospects for each play. Some specifications established distributions, while others were given as single values. From this information, sizes of oil and gas accumulations were simulated using a Monte Carlo algorithm. The number of such accumulations considered in a given simulation run was obtained from the distribution of the number of prospects. Each prospect in each successful simulation run was risked. This process yielded size-frequency distributions and summary statistics for the various petroleum categories. Estimates of remaining resources from individual plays were then aggregated, and measures of uncertainty computed. Technically recoverable, undiscovered oil beneath the Federal part of NPRA likely ranges between 5.9 and 13.2 billion barrels, with a mean (expected) value of 9.3 billion barrels. Technically recoverable, undiscovered nonassociated natural gas for the same area likely ranges between 39.1 and 83.2 trillion cubic feet, with a mean (expected) value of 59.7 trillion cubic feet. Mean values of the corresponding associated dissolved gas and natural gas liquid are 10.3 trillion cubic feet and 1.4 billion barrels respectively.