When properly designed, hypolimnetic aeration and oxygenation systems can replenish dissolved oxygen in water bodies while preserving stratification. The three primary devices are the airlift aerator, Speece Cone, and bubble-plume diffuser. In each device, gas bubbles in contact with water facilitate interfacial transfer of oxygen, nitrogen, and other soluble gases. However, early design procedures for airlift aerators were empirical, while most bubble-plume models did not account for stratification or gas transfer. Using fundamental principles, a discrete-bubble model was first developed to predict plume dynamics and gas transfer for a circular bubble-plume diffuser. The discrete-bubble approach has subsequently been validated using oxygen transfer tests in a large vertical tank and applied successfully at full-scale to an airlift aerator as well as to both circular and linear bubble-plume diffusers. The performance of each of the four completely different full-scale systems (on a scale of 10 m or more) was predicted based on the behavior of individual bubbles (on a scale of about 1 mm). The combined results suggest thatthe models can be used with some confidence to predict system performance based on applied air or oxygen flow rate, initial bubble size, and, in the case of bubble plume diffusers, near-field boundary conditions. The discrete-bubble approach has also been extended to the Speece Cone, but the model has not yet been validated due to a lack of suitable data. The unified suite of models, all based on simple discrete-bubble dynamics, represents the current state-of-the-art for designing systems to add oxygen to stratified lakes and reservoirs.