Magnetic Carnot cycle refrigerators should be capable of pumping heat efficiently and inexpensively from liquid helium temperatures to liquid hydrogen temperatures. At higher temperatures, up to room temperature, Stirling cycle devices using ferromagnets are feasible. At low temperatures paramagnets absorb hundreds of joules per liter upon demagnetization ; at higher temperatures, ferromagnets absorb tens of kilojoules per liter upon demagnetization. Four magnetic refrigerators have been built, but no economically viable unit is in operation. However, fundamental consideration indicates that magnetic refrigerators should eventually replace gas refrigerators, at least below about 80 K where lattice specific heats can be kept small.