Lithium (Li) concentrations of produced water from unconventional (horizontally drilled and hydraulically fractured shale) and conventional gas wells in Devonian reservoirs in the Appalachian Plateau region of western Pennsylvania range from 0.6 to 17 mmol kg−1, and Li isotope ratios, expressed as in δ7Li, range from +8.2 to +15‰. Li concentrations are as high as 40 mmol kg−1 in produced waters from Plio-Pleistocene through Jurassic-aged reservoirs in the Gulf Coast Sedimentary Basin analyzed for this study, and δ7Li values range from about +4.2 to +16.6‰. Because of charge-balance constraints and rock buffering, Li concentrations in saline waters from sedimentary basins throughout the world (including this study) are generally positively correlated with chloride (Cl), the dominant anion in these fluids. Li concentrations also vary with depth, although the extent of depth dependence differs among sedimentary basins. In general, Li concentrations are higher than expected from seawater or evaporation of seawater and therefore require water–mineral reactions that remove lithium from the minerals. Li isotope ratios in these produced waters vary inversely with temperature. However, calculations of temperature-dependent fractionation of δ7Li between average shale δ7Li (−0.7‰) and water result in δ7Liwater that is more positive than that of most produced waters. This suggests that aqueous δ7Li may reflect transport of water from depth and/or reaction with rocks having δ7Li lighter than average shale.