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The term ‘hypersonic flow’ was first used by Tsien in 1946 to denote a (low for which the main stream Mach number was large compared with unity, and he demonstrated that such flows displayed characteristic features which justified the use of a special name. Tsien confined his considerations to a perfect gas with constant specific heats but since 1946 interest has widened to the flow of real fluids at high Mach numbers, this interest being mainly stimulated by the problems of the re‐entry at high speeds into the earth's atmosphere of missiles and satellites. An essential feature of hypersonic flow is that relative to the undisturbed flow direction the inclination of the nose shock of a body immersed in the fluid is of the same order of magnitude as the mean inclination of the surface over the forward part of the body, and the region between shock and body, the so‐called ‘shock layer’, is relatively narrow there. Another characteristic feature is the high temperature that developes in this layer in problems of practical interest and the associated effects on the physical and chemical properties of the medium. Thus, not only must account be taken of the variation of the specific heats with temperature for a real fluid but the consequences of dissociation and ionization of the fluid on crossing the nose shock must be considered. The interaction of the boundary layer with the flow external to it and with the nose shock becomes of increasing importance, as well as increasingly complex, with increase of main stream Mach number. Finally, account must be taken of the molecular nature of the medium in problems where the density is sufficiently low for the mean free path of the molecules to be a significant ratio of a typical dimension of the body or of its boundary layer thickness.