The purpose of this chapter is to provide background information on the development and application of the rules for construction of elevated temperature components for nuclear service. Subsection NH of Section III, 2011a Addenda  contains the rules for Class 1 nuclear components consisting of vessels, pumps, valves, and piping systems. The rules for Class 2 and 3 Components and Core Support Structures are contained in a series of Code Cases [2–5]. The above rules cover all aspects of construction: materials, design, fabrication, inspection, overpressure protection, testing, and marking. In Section III, “elevated temperature” has been defined as 700°F for ferritic steels and 800°F for austenitic stainless steels and nickel base alloys. At and below these temperatures the rules for construction of Class 1, 2, and 3 Components and Core Support Structures are contained in Subsections NB , NC , ND , and NG , respectively, 2011a Addenda. Additionally, as discussed in Chapter 17, a new division of Section III, Division V has been issued to provide a central source for the construction rules for advanced high temperature reactors.The purpose of the early code cases for elevated temperature service, which subsequently became Subsection NH, was to provide rules for construction which account for the effects of deformation and damage due to creep with the same rigor that Subsection NB addressed the temperature regime below which creep effects are significant. In 1963, Code Case 1331 was issued covering design rules for elevated temperature Class 1 components. The early versions of Code Case 1331 were relatively limited in scope and it was not until Code Case 1331-5  was issued in 1971 that the basic design provisions of what is now Subsection NH were incorporated. In 1974 a series of Code Cases, 1592–1596 [12–16], were issued which covered all aspects of elevated temperature construction for Class 1 components. Subsequently, additional cases were issued covering Class 2 and 3 components and core support structures.In 1976, three articles [16–18] were published in the Journal of Pressure Vessel Technology which covered the development of the rules for construction (all aspects), design, and buckling, respectively, of Section III, Class 1 components in elevated temperature service. Also, in 1976 a document titled “Criteria for Design of Elevated Temperature Class 1 Components”  was issued that was analogous to the criteria document for Class 1 Components  covered by Subsection NB. These first four references form the core of much of the discussion in this chapter related to Class 1 components; modified and expanded to cover the many revisions and additions to the rules occurring after their publication. An excellent reference on the application of elevated temperature design criteria from a world-wide perspective is the four-volume set “Recommended Practices in Elevated Temperature Design: A Compendium of Breeder Reactor Experiences (1970–1987),” edited by A. K. Dhalla and issued by the Welding Research Council [21–24]. Although there were many contributing organizations in the United States and worldwide through exchange agreements and voluntary contributions, the focal point for development of the criteria in Subsection NH was the Oak Ridge National Laboratory. Unfortunately, there is no bibliography of their many reports and publications; however, certain of these are referenced in the applicable sections of this chapter.