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The flow fields encountered by jet- and fan-powered vertical/short takeoff and landing (V/STOL) aircraft when hovering in ground effect are reviewed and their effects on the aerodynamic characteristics are discussed. The ground effects considered include the suckdown generated by the flow from a single nozzle, the fountain effects generated by multiple-nozzle configurations, and the additional suckdown associated with the fountain flow generated by multiple-nozzle configurations. Our current understanding of the flow fields involved and the capability and limitations of available methods for estimating the effects of ground proximity are reviewed and the areas where additional work is needed are discussed.