A flow control actuator 10 for a body exposed to fluid flow comprises first and second flow surfaces 14, 16 spaced to define an elongate gap 20 therebetween. An elongate control element is disposed in or adjacent the gap and has an externally facing arcuate surface and defines a first slot 24 between the control element and said first flow surface and a second slot between said control element and said second flow surface 26. The control element 22 is moveable to allow the width of said slots to be adjusted generally proportionally. The flow of pressurized flow control fluid passes from a plenum chamber 30 out through one or both slots 24, 26 to be deflected around the arcuate surface under the influence of the Coanda effect. The flow control actuator 10 may be provided in the trailing or leading edge of an aerofoil; alternatively it may be provided between two surfaces spaced to provide said gap but otherwise providing a generally continuous surface.