The data-base for this study is mainly unpublished Mongolian and Former Soviet Union reports (1958 to 1989), and studies conducted by or for the Mongol Petroleum Co. (Mongol Gazryn Tos. MGT) (1989 to 1992). A very small amount of data was obtained from published literature Until 1989, the majority of work on the petroleum geology of Mongolia either condemned the prospects, or suggested that there was only very limited oil and gas potential However, the present evaluation, which is based on three years＇ work by the Author and his associates, supported by Mongol Petroleum Co. explorationists, is that three to six billion brls oil-equivalent (o.e.) of conventional recoverable oil and gas resources within the territory of Mongolia is a reasonable expectation. The expected ＇＇most likely＇＇ oilfield size ranges from 100 to 170 million (MM) brls o.e., with the ＇＇low＇＇ oilfield size ranging from 5 to 10 MM brls o.e. and the ＇＇high＇＇ oilfield size ranging from 330 to 700 MM brls o.e. The concepts upon which these estimates are based were derived from Klemme (1980, 1983, 1986); Klemme and Ulmishek (1991); Ulmishek (1986); and Ulmishek and Klemme (1990).