Oscillatory addition of momentum without the addition of mass flow is very effective in delaying separation, particularly from lifting surfaces. It is much more robust than the steady blowing traditionally used for this purpose. Experiments carried out on different airfoils revealed that this flow depends on many parameters such as: the location of the blowing slot, the steady and oscillatory momentum coefficients (if oscillatory blowing is used), the frequency of the imposed oscillations and the shape and incidence of the airfoils. The incremental improvements in the airfoil characteristics are insensitive to changes in Reynolds number provided the latter is sufficiently large. Preliminary results suggest that the improvements in the airfoil performance are not hindered by compressibility at subsonic speeds or by sweep. Furthermore the method can be applied to diffusers and thus used for thrust augmentation and vectoring. It can also be applied to helicopter rotors and alleviate the effects of dynamic stall, some preliminary results on pitching airfoil are shown. The oscillatory blowing and suction used most often can be replaced by other means providing the appropriate oscillatory component of the momentum coefficient at the right frequency. Theproducing the oscillations. It became clear that a delay of separation is a distinctly different task than the promotion of.reattachment yet both are important in operating an airfoil or a flap near its natural stall. Some examples, showing the instantaneous (or phase locked to the disturbance and ensemble-averaged) pressure distributions and voiticity distributions over a flap will be given. The integration of geometrical design with the imposed forced oscillations providing the maximum pressure recovery over the shortest possible distance is now under consideration.