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This chapter provides commentary on a new division under Section III of the ASME Boiler and Pressure Vessel (BPV) Code. This Division 5 was first published as part of the 2011 Addenda to the 2010 Edition of the BPV Code. The chapter provides information on the scope and need for Division 5, the structure of Division 5, where the rules originated, the basis for the elevated temperature rules specified in Division 5, the various changes made in finalizing Division 5, and the future near-term and long-term expectations for Division 5 development. It reflects the 2015 Edition of Division 5. Division 5 identifies rules for high temperature reactors (HTRs) based on only two classifications, Class A for safety-related components and Class B for non-safety related but with special treatment components. It contains general requirements for both metals and graphite in Subsection HA, Subpart A and Subpart B, respectively. Rules for Class A metallic pressure boundary components, Class B metallic pressure boundary components, and Class A core support structures at both low temperature conditions (under Subpart A) and elevated temperature conditions (under Subpart B) are contained in Subsections HB, HC, and HG, respectively. Rules for Class A and B metallic supports are contained in Subsection NF, Subpart A. Finally, new rules for nonmetallic core support structures (graphite) are contained in Subsection HH, Subpart A. Consistent with current Code practice, the primary concern of Division 5 is the integrity of the components under design, operating conditions (including normal, upset, emergency, and faulted), and test conditions. Division 5 covers all construction aspects of these components, including administrative requirements, material selection and qualification, design, fabrication, examination, inspection, testing, quality assurance, and documentation.