The development and validation of new technology for the accurate and traceable metering of high value fluids is presented here. The focus of this doctoral submission is on the determination of fluid density by the measurement of relative permittivity. A prototype cell, comprising a re-entrant cavity resonator, for the precise determination of the relative permittivity of gases and hydrocarbon liquids over a wide range of both pressures and temperatures has been developed for this work. Accuracies of measurement of relative permittivity with the re-entrant cavity resonator technique of better than 1 ppm may be achieved. Reference quality relative permittivity measurements were performed and expressions developed for ethylene which are specific to industrial metering applications ( 0 ≤ t ≤ 30°C and 5 ≤ p ≤ 10 MPa). The uncertainty in values of density calculated from the mapping relationship is approximately 0.03 % in density at a 95 % confidence level. The laboratory facility used to perform the fluid mapping or characterisation was based around a high-performance RF network analyser as the principle measuring instrument. However, an on-line instrument must be simple to operate, relatively compact, robust and considerably less expensive; particularly if it is to be widely deployed. The aimed accuracy in the measurement of relative permittivity of the on-line instrument was 5 ppm; a factor of five lower than the laboratory instrument. For the on-line instrument, the re-entrant cavity resonator was incorporated into a feedback oscillator circuit as the frequency determining element. The accuracy of measurement of relative permittivity of the on-line instrument was 2.5 ppm; a factor of two greater than the aimed accuracy. This accuracy of frequency measurement is only achievable over a relatively narrow range of operating conditions, which is ultimately a limiting factor in the applicability of the on-line instrument for high precision relative permittivity measurements in the field.