The Background Oriented Schlieren (BOS) technique is a method that has been developed recently for easily and inexpensively visualizing density gradient variations in compressible flows. In this thesis, the applications of this technique in the Drummond tunnel and the T4 free-piston reflected shock tunnel are presented. The BOS technique was first tested in the Drummond tunnel then used during four test campaigns to qualitatively visualize the flow over scramjet models and components of scramjets in the T4 shock tunnel. The results demonstrate that a simple optical arrangement, that does not need high quality windows in the test section and model, can be used to detect the locations of shock waves, expansion fans and shear layers. At present, the resolution and quality of the images is not as high as can be achieved using conventional Schlieren techniques.