This report discusses the use of high-speed Schlieren to study an under-expanded jet impinging on a flat plate. Images were acquired over a range of nozzle pressure ratios (1.7 ≤ NPR ≤ 2.5) representing high-subsonic and mildly under-expanded flow. Mean pressure measurements and averaged Schlieren images indicated that no recirculation bubble was present in the impingement zone of the jet. Sequences of Schlieren images showed a complex interaction between large-scale vortical structures and normal shocks within the jet. The phenomenon known as shock splitting was observed. Instantaneous images showed that for NPR ≤ 2.1 an axisymmetric instability mode dominated the jet while for higher NPRs the instability became asymmetric, either sinusoidal or helical. Moving the nozzle closer to the plate decreased the degree of unsteadiness in the jet shear layer due to the constraint on the growth of large-scale vortices. For h/d = 1, no large scale vortices were observed in the jet shear layer. For h/d = 4, near the nozzle exit an axisymmetric periodic expansion/contraction cycle of the jet was observed at low NPR. Quantitative analysis of the images indicated that the frequency of this cycle was approximately one order of magnitude below the initial instability frequency of the jet. Results suggest that this order of magnitude reduction in frequency is potentially the result of 'collective interaction' which involves simultaneous pairing of several vortices.