The high (9-12%) chromium steels are either fully austenitic or have a duplex (austenite plus δ-ferrite) structure at austenitizing temperatures in the range 850 to 1200°C. The austenite phase transforms to martensite during air cooling or rapid quenching to ambient temperature, and the steels are subsequently tempered to obtain a good combination of strength, ductility, and toughness. The physical metallurgical basis of the development of the high-chromium martensitic steels has been reviewed elsewhere [1-3]; the factors determining the constitution, transformation, and tempering characteristics are briefly surveyed in this chapter.