Boilers are often classified based on the maximum operating temperature and pressure for which they are designed. Classifications, in ascending order, are subcritical, supercritical, ultra-supercritical, and to advanced ultra-supercritical. At each higher operating point comes greater efficiency, as well as greater demand on construction materials. This chapter discusses the primary requirements for boiler tube materials, including oxidation and corrosion resistance, fatigue strength, thermal conductivity, and the ability to resist creep and rupture. It also provides information on various steels and alloys, covering cost, engineering specifications, and ease of use.