This paper reviews the work on applying the second law to cryogenics, and the benefits that have been derived from this application. The need to continue the use of the second law in the design of future cryogenic systems is discussed. Techniques of applying the second law to systems analysis developed in the cryogenic field are reviewed. The second law of thermodynamics has been more extensively applied in the cryogenic field than in other engineering areas, primarily due to the low Carnot efficiencies involved with cryogenic temperature systems. Early applications of second law analysis resulted in using staged or cascaded systems and work recovery devices to reduce the power requirements for air and helium liquefaction systems. More recent second law studies, based on minimization of irreversible entropy production and exergy losses, are providing a greater insight into the influence of irreversible losses on the power expended to achieve cryogenic temperature refrigeration. These studies show the significant penalty in power attributed to irreversible system losses, and also indicate some design and operational procedures that can minimize this power penalty. Some future energy systems, such as magnetohydrodynamic power, super conducting magnet energy storage, fusion power, magnetic levitated transportation, LNG, and liquid hydrogen storage and transportation involve cryogenic technology. These systems will benefit from consideration of the second law in their design and performance analysis.