Abstract. The development and design of a cryogenic liquid quantity gauge for zero-g applications is described . The gauge, named the Compression Mass Gauge (CMG), operates on the principle of slightly changing the vo lume of the tank by an oscillating bellows. The resulting pressure change is measured and used to predict the volume of vapor in the tank, from which the volume of liquid is computed. For each gauging instance, pressures are measured for several different bellows frequencies to enable minor real-gas effects to be quantified and thereby to obtain a gauging accuracy of ± 1 % of tank volume. Southwest Research Institute™ and NASA-GRC have developed several previous breadboard and engineering development gauges and tested them in cryogenic hydrogen and nitrogen to establish the gauge capabilities, to resolve several design issues, and to formu late data process ing algorithms. The CMG has been selected by NASA's Future X program for a fli gh t demonstration on the USAFlBoeing Solar Thermal Vehicle Space Experiment (SOTVSE). This paper reviews the design trade studies needed to satisfy the SOTVSE limitations on CMG power, volume, and mass, and describes the mechanical design of the CMG.