This study compares the additions to conventional crude oil and natural gas reserves as reported from January 1996 to December 2003 with the estimated undiscovered and reserve-growth volumes assessed in the U.S. Geological Survey World Petroleum Assessment 2000, which used data current through 1995. Approximately 28% of the estimated additions to oil reserves by reserve growth and approximately 11% of the estimated undiscovered oil volumes were realized in the 8 yr since the assessment (27% of the time frame for the assessment). Slightly more than half of the estimated additions to gas reserves by reserve growth and approximately 10% of the estimated undiscovered gas volumes were realized. Between 1995 and 2003, growth of oil reserves in previously discovered fields exceeded new-field discoveries as a source of global additions to reserves of conventional oil by a ratio of 3:1. The greatest amount of reserve growth for crude oil occurred in the Middle East and North Africa, whereas the greatest contribution from new-field discoveries occurred in sub-Saharan Africa. The greatest amount of reserve growth for natural gas occurred in the Middle East and North Africa, whereas the greatest contribution from new-field discoveries occurred in the Asia Pacific region. On an energy-equivalent basis, volumes of new gas-field discoveries exceeded new oil-field discoveries. Tim Klett is a research geologist at the U.S. Geological Survey, where he is responsible for assessments of undiscovered petroleum resources worldwide. He received his Ph.D. in geology and geochemistry from the Colorado School of Mines. Prior to joining the U.S. Geological Survey in 1991, he worked as a mud logger and later, in 1984, joined Exxon Production Research Company (now ExxonMobil Research Company).Donald L. Gautier is a geologist at the U.S. Geological Survey. His current research focuses on the resource potential of the Arctic, and he recently completed a study of the San Joaquin basin. Born in Los Angeles, Gautier holds a Ph.D. in geology from the University of Colorado and worked for Mobil before joining the U.S. Geological Survey in 1977. Thomas Ahlbrandt received his B.A. degree (1969) and his Ph.D. (1973) in geology at the University of Wyoming. He has 19 years of industry experience in exploration and research with ExxonMobil, BP Amoco, Amerada Hess, and independents and 20 years of service with the U.S. Geological Survey. He currently is the World Energy Project Chief in Denver and led the 2000 U.S. Geological Survey World Petroleum Assessment.