Ames Laboratory scientists who discovered the giant magnetocaloric effect (GMCE) about 15 years ago now have unlocked the mystery associated with the giant increase in the MCE. They have devised procedures to determine the structural entropy contribution, ▲Sst (which is difficult to measure directly), to the total entropy change of the GMCE, ▲ST (see upper set of figures). Analysis of the GMCE observed in 5 different systems shows that the structural entropy change is proportional to the volume difference between the paramagnetic and ferromagnetic phases which have different crystal structures (see lower figure). The larger the GMCE – the greater the cooling power and the higher the efficiency of magnetic refrigeration. Since structural information is generally known it is possible to estimate the entropy change for a GMCE material by using crystallographic data, i.e. the next advanced magnetocaloric compound may be discovered in a scientist’s office and not in the laboratory.