Sonoluminescence is the phenomenon whereby acoustic energy is converted into light through the violent collapse of gas bubbles in a liquid. A salient feature of sonoluminescence is the emission of a blackbody spectrum. Recent experiments have shown that the flashes of light from sonoluminescence are generated from a dense plasma characterized by an unusually high electron density. Furthermore, this plasma is a new state of matter whose properties are derived from a thermodynamic equation of state and is independent of how it was generated. In this dissertation, the universality of sonoluminescence is tested and its plasma properties are explored. Highly-ionized dense plasmas are confirmed in two different sonoluminescence systems using new experimental techniques. Plasmas are generated in high-pressure gases for two different systems whose properties are remarkably similar to sonoluminescence in temperature and ionization. Taken together, these experiments show that the plasma of sonoluminescence is not unique to itself and is consistent with a newly-discovered phase of matter.