Flow control in the modern meaning of the term, is a technology that enables gains in performance, greater than that achievable using conventional design tools. To maximise the potential of this technology, forms of actuation have to be developed that require low power, do not modify the structure of the vehicle considerably, are reliable and above all efficient and effective in controlling the flowfield. A promising form of actuation is known as the synthetic or massless jet, so called because for an axi-symmetric orifice, a jet is formed from a train of vortex rings, with zero net mass flux. The synthetic jet actuator often consists of an orifice plate from which the vortex rings are formed, mounted to a cavity with a diaphragm at the other end. The periodic oscillation of the diaphragm causes the roll-up of a vortex ring at the orifice exit, that for sufficient levels of forcing, convects away under its self-induced velocity, before the next suction stroke commences. The present research is focused upon the fundamental understanding of the structure of an axi-symmetric synthetic jet embedded in a turbulent boundary layer, with the aim of understanding how non-dimensional parameters of practical importance affect the structure and the dynamics of the synthetic jet formation. A series of basic experiments that gradually added the salient features of an embedded synthetic jet were undertaken, and demonstrated that parameters such as the jet velocity ratio and Strouhal number have a large effect upon the dynamics and structure of a synthetic jet subjected to quiescent conditions, a cross-flow and shear in the form of a turbulent boundary layer. A model of a synthetic jet embedded in a boundary layer has been proposed based upon the results of these experiments, and hypothesises that the ejected vortex rings form two periodic counter-rotating streamwise vortical structures, provided that the vortex rings do not penetrate beyond the boundary layer. The effectiveness of synthetic jets in delaying the separation of a turbulent boundary layer from a circular cylinder by up to 5% in azimuth, was demonstrated. Surface flow visualisation provided further evidence of the presence of two periodic counter-rotating streamwise structures, and that three-dimensionality of the boundary layer near to separation, has a large effect on the structure of the synthetic jet and its interaction with the separation.