The effect of subzero treatment on tool steels is well known, and is commercially practiced for certain varieties of tool steels. Substantial research is being carried out on the behavior of tool steels after cryogenic treatment (CT). While it has been established that CT improves the wear properties in some steels, the mechanism of this behavioral aspect is not yet fully known. Most work has been focused on AISI M2 high speed steel related to the effect on different properties, but the microstructural change is not yet conclusively identified. Neither the past research has brought forth the exact principles on which CT should be based. Some of these important issues are addressed in this paper.