Our group carries out studies in low-temperature physics on the properties of superfluid helium. Currently dilution-refrigerator experiments are in progress on the superfluid phase transition of helium films adsorbed in porous materials, to examine the role of quantized vorticity in this three-dimensional transition. Based on these results, a theory of the bulk superfluid lambda-transition has been formulated using vortex ring excitations. A new project is to study the properties of sonoluminescence at cryogenic temperatures is being undertaken. We have observed very bright sonoluminescence with xenon gas in ethanol at -115 C, and further measurements will be tried in other cryogenic liquids such as nitrogen, neon, and helium.