Various controversial issues relating to the definition and interpretation of the term proven reserves are cited and discussed. Furthermore, reserve figures published in oil industry periodicals and trade journals are reviewed and analyzed. Original reserves (reserves plus cumulative production) of non-OPEC countries seem to increase almost linearly with time, especially since the early 1980s, with an average rate of rise of 15 billion barrels per annum. The OPEC trend, however, is rather distorted by irregular revisions, and the rate of additions to reserves in non-OPEC countries seems to have surpassed OPEC countries during the 1990s. Nevertheless, OPEC, with its easily accessible low-cost oil, will very likely continue to be the paramount source of world oil for many decades.