Any estimation of resource quantities is subject to uncertainty and should thus be expressed as a potential range. The function of the three primary categories of reserves (proved, probable, possible) is to illustrate the range of uncertainty in the estimate of the potentially recoverable volume of petroleum from a known accumulation. Such estimates, which are done for each well or reservoir, may be made according to the deterministic or the probabilistic methods and are then aggregated for the whole accumulation/project. The estimate of uncertainty for each accumulation can be tracked over time from exploration through discovery, development and production. This approach provides an extremely effective basis for evaluating the validity of the methodologies used.