The Upper Jurassic Haynesville Shale is currently regarded as one of the most prolific emerging shale-gas plays in the continental United States. It has estimated play resources of several hundred trillion cubic feet and per-well reserves estimated as much as 7.5 bcf. The reservoir spans more than 16 counties along the boundary of eastern Texas and western Louisiana. Although this basin has a long history of exploration and analysis of its Mesozoic section, a comprehensive subsurface study characterizing the Haynesville Shale has not been conducted. This article is the first to address the structural setting, stratigraphy, depositional environment and facies, fracturing, and production challenges of the Haynesville shale-gas play. Basement structures and salt movement influenced carbonate and siliciclastic sedimentation associated with the opening of the Gulf of Mexico. The Haynesville Shale is an organic- and carbonate-rich mudrock that was deposited in a deep partly euxinic and anoxic basin during the Kimmeridgian to the early Tithonian, related to a second-order transgression that deposited organic-rich black shales worldwide. The Haynesville Basin was surrounded by carbonate shelves of the Smackover and Haynesville lime Louark sequence in the north and west. Several rivers supplied sand and mud from the northwest, north, and northeast into the basin. Haynesville mudrocks contain a spectrum of facies ranging from bioturbated calcareous mudstone, laminated calcareous mudstone, and silty peloidal siliceous mudstone, to unlaminated siliceous organic-rich mudstone. Framboidal to colloidal pyrite is variably present in the form of concretions, laminae, and individual framboids and replaces calcite cement and mollusk shells. Haynesville reservoirs are characterized by overpressuring, porosity averaging 8 to 12%, Sw of 20 to 30%, nanodarcy permeabilities, reservoir thickness of 200 to 300 ft (70 to 100 m), and initial production of as much as 30 mmcf/day. Reservoir depth ranges from 9000 to 14,000 ft (3000 to 4700 m), and lateral drilling distances are 3000 to 5000 ft (1000 to 1700 m). Typical Haynesville wells exhibit a steeper decline curve (similar to 80% in the first year) than other shale-gas plays, which is attributed to a very high overpressure.