Most of the information on ferritic/martensitic steels for nuclear applications comes from studies on commercial Cr-Mo steels, primarily 9-12% Cr, 1-2% Mo, 0.1-0.2% C with small amounts of V, Nb, W, Ni, etc. (Compositions throughout the book will be in wt% unless otherwise stated.) These were the ferritic steels considered first for fast breeder fission reactors in the early 1970s and then in the late 1970s for fusion applications. The steels became of interest because of their swelling resistance compared to austenitic stainless steels, which were the primary candidates for both applications up to that time [1,2].