Recent results on the atomization and breakup of cryogenic propellants under high-pressure subcritical and supercritical conditions are surveyed. Cryogenic  uids were injected into various gases under both cold- ow and hot-Ž re conditions, and the results were visualized using  ashlight photography and highspeed cinematography. In some cases, simulation  uids were used to study atomization and breakup behavior. In others, hot-Ž re tests with liquid oxygen and gaseous hydrogen demonstrated  ow phenomena under realistic cryogenic rocket engine conditions. The visualizations reveal a remarkable difference between subcritical spray formation and evaporation and supercritical injection and mixing. As chamber pressure approaches the critical pressure, injection can no longer be regarded as simple ‘‘spray’’ formation, but rather as a  uid/ uid mixing process that can be extremely sensitive to small perturbations in pressure, temperature, local mixture concentrations, and initial injection conditions.