In this paper the physics of plasma expansion in the rarefied regime is reviewed. Densities, temperatures, and velocity distributions in argon, hydrogen, and nitrogen expansions that have been measured using laser scattering and fluorescence techniques are compared. The velocity distributions in the region of the expansion where the density is below the background density show a bimodal character. It is interpreted in terms of a component expanding from the source and a component flowing into the plasma expansion from the periphery. Also in the shock of the expansion, bimodal velocity distributions are encountered. These distributions show the gradual change in the flow from supersonic to subsonic—the formation of the shock. From a comparison of the three expansions, a general view of the shock formation is derived. This new insight leads to a better understanding of how the chemical reactivity of the usually impenetrable, supersonic plasma can be used most efficiently.