Hypersonics is the study of flight at speeds where aerodynamic heating dominates the physics of the problem. It is an engineering science with close links to supersonics and engine design. Within this field, many of the most important results have been experimental. The principal facilities have been wind tunnels and related devices, which have produced flows with speeds up to orbital velocity. Why is this important? Hypersonics has had two major applications. The first has been to provide thermal protection during atmospheric reentry. Success in this enterprise has supported ballistic-missile nose cones, has returned strategic reconnaissance photos from orbit and astronauts from the Moon, and has even dropped an instrument package into the atmosphere of Jupiter. The second application has involved high-speed propulsion and has sought to develop the scramjet as an advanced airbreathing ramjet. Atmospheric entry today is fully mature as an engineering discipline, but work with its applications continues to reach for new achievements. Studies of scramjets still seek full success, in which such engines can accelerate a vehicle without the use of rockets. Hence, there is much to do in this area as well.