The prime need to know Mach number in flight is likely to be for the efficient operation and control of air-breathing propulsion units. The errors incurred by the use of the analytical expressions, which have been developed for supersonic flight, are quantified and shown to be increasingly serious for speeds in excess of M = 5.5 approximately. Systems for calculating this parameter in flight are reviewed and compared. It is shown that three out of the four systems identified are capable, potentially, of registering Mach number to a good precision. In each case, to achieve optimum precision requires a good knowledge of ambient air temperature. Such knowledge is likely to be available from meteorological satellites.