Because of the ever-present problem of heat leak to a cryogenic fluid transfer system, two-phase flow and boiling heat transfer are not uncommon in these systems. Two-phase flow is always encountered in cryogenic liquid gasifiers (vaporizers) used in such applications as self-pressurization systems and LNG (liquid natural gas) gas delivery systems (Filina and Weisend 1996). In this chapter, the phenomena of two-phase flow, including techniques for prediction of the flow regimes and pressure drop in two-phase flow, will be examined. Physical principles associated with both pool boiling (boiling on a surface immersed in a cryogenic liquid) and forced convection boiling (boiling within flow passages) will be presented. Two-phase flow may involve condensation instead of boiling, and condensation heat transfer in both external flow and internal flow will be considered. Finally, some problems involving solid–liquid phases (such as slush hydrogen) will be examined.