Hypolimnetic aeration is an increasingly common management technique that aerates the hypolimnion while preserving thermal stratification. While most hypolimnetic aeration systems use air as an oxygen source, use of pure oxygen is growing. Potential benefits of hypolimnetic oxygenation include maintenance of an oxygenated source of cool water to meet consumer and environmental needs, decreases in internal nutrient loading, inhibition of sediment release of problematic reduced compounds, and maintenance of summertime habitat for cold-water fish, zooplankton and zoobenthos. A number of short-term experimental oxygenation systems were operated in the 1970s, but large- scale systems were not implemented until the 1980s. Deep oxygen injection systems have been operating in Lakes Sempach, Baldegg, and Hallwil, Switzerland, since the early 1980s to ameliorate cultural eutrophication. Deep oxygen injection has also been used to increase DO in hydroelectric releases from a number of large reservoirs in the southern USA A comprehensive study of physical, chemical and biological impacts of deep oxygen injection was performed at Amisk Lake, Alberta, from 1988-1993. In the early 1990s, downflow oxygen bubble contact chambers (Speece Cones) were installed in Newman Lake, Washington, and Camanche Reservoir, California, mainly to improve the quality of cold- water fishery habitat. Compared to hypolimnetic aeration, oxygenation results in higher hypolimnetic dissolved oxygen levels, lower levels of induced oxygen demand, and maintenance of more stable thermal stratification. Operational experiences over the past two decades confirm that hypolimnetic oxygenation is a successful management strategy with numerous water quality benefits.