In 1998, a new assessment of the geology and hydrocarbon resources in part of the Arctic National Wildlife Refuge (ANWR) in Alaska was prepared by the U.S. Geological Survey and is-here summarized. The ANWR is located between the petroleum-rich provinces of Prudhoe Bay to the west and the Mackenzie delta to the east. The new assessment is focused on the coastal-plain part of the ANWR (the so-called 1002 area), where hydrocarbon trends found in these adjacent areas are believed to exist. Significant new information from recent wells, field studies, laboratory analysis, and newly reprocessed and reinterpreted geophysical data support the identification of 10 major petroleum plays in the 1002 area. Four of these plays, identified by seismic facies mapping as topset, turbidite, wedge, and thin-skinned thrust belt plays, hold more than 85% of the interpreted total resources. These plays exist in Upper Cretaceous-Tertiary foreland basin strata (Brookian sequence) and can be related to three petroleum systems, which suggest that oil was generated, expelled, and entrapped mainly between the Eocene and Miocene. Total estimated volumes for the 1002 area are in-the range of 11.6-31.5 billion bbl oil (95th to 5th percentile probability) for in-place resources and 4.8-11.8 billion bbl for technically recoverable resources. Because no drilling has occurred in the ANWR itself, these estimates must be understood as hypothetical. The volumes noted represent a significant increase over those derived by an earlier assessment of the same area jointly conducted by the Bureau of Land Management and the U.S. Geological Survey.