Book excerpt: An experimental study was conducted to examine some fundamental aspects of the motion of single noncondensable bubbles under low gravitational conditions. An acceptable criterion was found for predicting the onset of unstable bubble motion which was consistent with published stability calculations at normal gravity, Data are also presented on the changes of frequency and amplitude of unstable (oscillatory) motion that occur in low gravity. The findings are applicable to bubbles in low-viscosity isothermal fluids at both normal and reduced gravity.