The Mississippian Barnett Shale serves as source, seal, and reservoir to a world-class unconventional natural-gas accumulation in the Fort Worth basin of north-central Texas. The formation is a lithologically complex interval of low permeability that requires artificial stimulation to produce. At present, production is mainly confined to a limited portion of the northern basin where the Barnett Shale is relatively thick (〉300 ft; 〉92 m), organic rich (present-day total organic carbon 〉 3.0%), thermally mature (vitrinite reflectance 〉 1.1%), and enclosed by dense limestone units able to contain induced fractures. The most actively drilled area is Newark East field, currently the largest gas field in Texas. Newark East is 400 mi2 (1036 km2) in extent, with more than 2340 producing wells and about 2.7 tcf of booked gas reserves. Cumulative gas production from Barnett Shale wells through 2003 was about 0.8 tcf. Wells in Newark East field typically produce from depths of 7500 ft (2285 m) at rates ranging from 0.5 to more than 4 mmcf/day. Estimated ultimate recoveries per well range from 0.75 to as high as 7.0 bcf. Efforts to extend the current Barnett play beyond the field limits have encountered several challenges, including westward and northward increases in oil saturation and the absence of lithologic barriers to induced fracture growth. Patterns of oil and gas occurrence in the Barnett, in conjunction with maturation and burial-history data, indicate a complex, multiphased thermal evolution, with episodic expulsion of hydrocarbons and secondary cracking of primary oils to gas in portions of the basin where paleotemperatures were especially elevated. These and other data imply a large-potential Barnett resource for the basin as a whole (possibly 〉200 tcf gas in place). Recent assessment by the U.S. Geological Survey suggests a mean volume of 26.2 tcf of undiscovered, technically recoverable gas in the central Fort Worth basin. Recovery of a significant portion of this undiscovered resource will require continued improvements in geoscientific characterization and approaches to stimulation of the Barnett reservoirs. Scott L. Montgomery is a petroleum consultant and author residing in Seattle, Washington. He holds a B.A. degree in English from Knox College (1973) and an M.S. degree in geological sciences from Cornell University (1978). He is the lead author for the Petroleum Frontiers monograph series, published by IHS Energy Group, and has published widely on topics in the petroleum industry.Dan Jarvie is an analytical and interpretive organic geochemist. He has worked on conventional petroleum systems and, since 1984, unconventional oil and gas systems as well. His work on the Barnett Shale spans much of the last decade. He earned a B.S. degree from the University of Notre Dame and was mentored in geochemistry by Don Baker of Rice University and Wallace Dow. He is president of Humble Geochemical Services. Kent drilled his first Barnett well in 1997 while employed with Chevron U.S.A., as a member of the company's Nonconventional Gas Business Team. He subsequently transferred to Mitchell Energy, where he was involved in the drilling of more than a thousand Barnett wells. Currently, Kent is with Star of Texas Energy Services, Inc., which is also active in the Barnett Shale play. He is a certified and registered (Texas and Wyoming) petroleum geologist. Richard Pollastro received an M.A. degree in geological science from the State University of New York at Buffalo in 1977. He joined the U.S. Geological Survey in 1978 and has served as a province geologist on the national and world energy assessment projects. His recent accomplishments include petroleum system analysis and resource assessments of the Bend arch–Fort Worth basin with particular focus on Barnett Shale, South Florida basin, and the Arabian Peninsula.