In Chapter 6, the analysis of convection heat transfer problems was considered; however, the fluid was treated as a continuum or continuous material. In many cryogenic heat transfer systems, such as multilayer insulations, the pressure of the gas is low enough for the gas molecules to be spaced sufficiently far apart that the continuum model is no longer accurate. The mechanisms for fluid flow and heat transfer are different for systems involving a vacuum than for systems with fluids at ambient or above ambient pressures. In this chapter, the principles of fluid flow and heat transfer in vacuum systems will be considered.