Cryogenic fluids are the best of coolants and they are the worst. Because of their very low temperature, they have excellent capacity to cool warmer fluids; and because of their high density in the liquid state, they provide for compact storage. However, when applied to the moderate temperature demands of environmental thermal control and life support systems, they present very unique challenges.These unique challenges are presented and Hamilton Sundstrand's experiences and solutions to these technical challenges are discussed. Results from past programs, testing of current space station hardware, and research experiences and findings in these areas are presented. In addition, future challenges in the use of cryogenic fluids in environmental systems are discussed.