The extension of manned, maneuvering flight into the Mach 2–3 regime disclosed several serious inlet/engine compatibility problems not adequately predicted by a priori analysis or laboratory testing. Although acceptable solutions have been found for these problems within existing aircraft, it is anticipated that the extension of operational flight to Mach 3.5 or 4.5 by future generations of aircraft will amplify presently known compatibility problems and may very well uncover new problems. This paper discusses several fundamental compatibility problems of the inlet/engine system: airflow matching, pressure distortion, and unsteady flow. In all cases, increasing design Mach number increases the severity of these problems and will place greater demands upon all components of the propulsion system. Close cooperation between the designers of the inlet and engine will be required to avoid the compatibility pitfalls of independently designing each component for peak performance. In all probability, compromise to both inlet and engine will be required to achieve acceptable stability and maximum system performance.