In recent years, the concept of flow-field diagnostics has become associated with methods to view and interpret complex flow structure. In fluid mechanics, the challenge has been to develop an understanding of unsteady motion, including vorticity, turbulence, shear flows, mixing phenomena, boundary layer evolution, and shock interactions. In combustion, issues of species mole fractions, reaction phenomena, mixing versus stirring, tem­ perature fields, flame zones, and nonequilibrium phenomena become impor­ tant. Over the past several years there has also been a growing interest in observing effects associated with the addition of energy to high-speed flows for drag reduction, shock control, and control of transition.