Thermal aging after the initial normalizing (or quenching) and tempering treatments (discussed in Chapter 3) produces additional precipitation and interfacial segregation of trace impurities and alloying elements, which can significantly affect the toughness and susceptibility to cracking and high-temperature deformation of the high-chromium martensitic steels. The current understanding of the effects of thermal aging with and without simultaneous stress application and the factors that influence the toughness and fracture behavior and creep-rupture characteristics are reviewed in this chapter.