A brief exposition (descriptive rather than quantative) is given of some of the common phenomena of compressible flow. The aim of the paper is to supply background material to the physicist who may wish to know a little more about the problems and techniques involved in the mechanics of compressible fluids. A graphical representation of a point source moving through a fluid at various velocities from subsonic to supersonic is given and described. There is also a description with sketches of the principal optical methods of wind-tunnel testing for compressible flows. A number of actual test photographs is included which show the type of picture obtained by the various test methods and which also show the formation of shock and Mach waves.