As flight speed increases into the hypersonic regime, the stagnation pressure and temperature inside the engine become so great that, for practical structures of acceptable mass, the flow must pass through the engine at supersonic, rather than subsonic, speeds, hence the term 'scramjet' (supersonic combustion ramjet). The classic application for the scramjet is to the long-range airliner offering extended hypersonic flight. It is also widely accepted as a possible complement to the rockets conventionally used for space launchers. This paper explores the topic of how scramjets may best be used and is the first Frederick S. Billig Lecture in Hypersonics of the International Society for Air Breathing Engines, to be given at the 14th ISABE Symposium,