Creep is defined as the time-dependent plastic deformation that occurs under the influence of a constant applied stress (although most tests are conducted with a constant load). At elevated temperatures, it is referred to as thermal creep. Deformation processes that operate during thermal creep include (1) dislocation climb in combination with dislocation glide that leads to slip, (2) dislocation climb that leads to subgrain formation, (3) grain boundary sliding, and (4) grain shape change by diffusional processes. The first two processes are the most important for the thermal creep of the high-chromium ferritic/martensitic steels at normal operating temperatures (600°C).