A number of methodologies are described that provide estimates of volumes of undiscovered hydrocarbons. These include the direct assessment of undiscovered volumes of oil and gas and statistical methodologies that extrapolate the expected volumes to be found by future drilling from past performances. The latter include finding-rate studies expressed as decline curves and as cumulative resource curves, both of which provide a measure of undiscovered volumes to be found with additional drilling. Another technique described is the application of probability theory to the aggregate of a number of probability distributions under different dependency assumptions and the comparison of these results to those obtained by the use of Monte Carlo aggregation techniques. In the assessment of undiscovered amounts of oil and gas, where the estimates are based on little direct knowledge, the probability theory aggregation methodologies look very promising. The assessments of the United States, of the entire world, and of wilderness lands are discussed.