The Monte Carlo Method has been increasingly used in the petroleum industry as a means of quantifying uncertainty. Most commonly, this technique is used to calculate a range of values for hydrocarbon volumes originally in place. The distributions of the input variables input into the Monte Carlo simulator are estimated by obtaining a sufficient number of measurements of reservoir and fluid properties. This report will analyze several depleted or very mature fields for which the ultimate hydrocarbon recovery is known. For each of these fields, distributions of porosity, water saturation, reservoir thickness, and reservoir size are obtained from cores, openhole logs, seismic, and other data. These distributions are input into the Crystal Ball computer program to obtain the cumulative distribution function (CDF) and probability density function (PDF) of oil or gas volume originally in place for each of the fields. By comparing the actual ultimate recovered volumes of hydrocarbons to the range of original in place volumes predicted by the Monte Carlo calculations, the accuracy of the Monte Carlo method will be analyzed.