This chapter begins with a short overview of the historical development of the ferritic/martensitic steels for nuclear energy systems, followed by a section that outlines the numerous nuclear applications where this type of steels are currently used or foreseen in future applications. Deleterious effects of water-reactor environments on cracking, complex processes corrosion and compatibility, and liquid metal embrittlement phenomena are presented in the next two sections and followed by a comprehensive presentation of the mechanical properties of the high-chromium tempered martensitic steels in the unirradiated and irradiated conditions. The specific issue of helium accumulation and its impact of tensile and fracture in the low-temperature regime is then discussed as well as the high-temperature embrittlement. Void swelling and irradiation creep are treated at the end of the chapter, which is closed by a statement on the future development of these steels to improve their performance.