Geothermal energy resources in North Dakota and South Dakota occur as low (T 130 mW m−2) in the discharge area in south-central South Dakota and anomalously low (≍30 mW m2) in the recharge area near the Black Hills and along the western limb of the Kennedy Basin in western South Dakota. Heat-flow disturbances are the result of vertical groundwater flow through fractures in the discharge area of the regional flow system in South Dakota are minor and may be significant only in deeply incised stream valleys. An important factor that controls the temperature of the resource in both North Dakota and South Dakota is the insulating effect of a thick (500–2000 m) layer of low thermal-conductivity shales that overlie the region. The effective thermal conductivity of the shale layer is approximately 1.2 W m−1 K−1 in contrast to sandstones and carbonates, which have conductivities of 2.5 to 3.5 W m−1 K−1. This low conductivity leads to high geothermal gradients (dT/dz > 50°C km−1), even where heat flow has normal continental values, that is 40–60 mW m−2. Engineering studies show that geothermal space heating using even the lowest temperature geothermal aquifers (T ≍ 40 °C) in North Dakota and South Dakota is cost effective at present economic conditions. The Inyan Kara Formation of the Dakota Group (Cretaceous) is the preferred geothermal aquifer in terms of water quality and productivity. Total dissolved solids in the Inyan Kara Formation ranges from 3,000 to more than 20,000 mg L−1. Porosities normally are higher than 20%, and the optimum producing zones generally are thicker than 30 m. The estimated water productivity index of a productive well in the Inyan Kara Formation is 0.254179 l s−1 Mpa−1. Deeper formations have warmer waters, but, in general, are less permeable and have poorer water quality than the Inyan Kara.