This chapter provides a chronological overview of the development of Section XI, from its inception in 1968 to the 2015 edition. It traces the development, edition-by-edition, of important Code elements, including the philosophy behind many of the revisions. Emphasis is placed on the 2003 Addenda through 2008 Addenda because they apply to a majority of nuclear plants in the United States and in other countries. Through an extensive tabulation of Code Interpretations, the chapter also attempts to give the Code user some insights into clarification of many Section XI requirements. Section XI has Divisions 1, 2, and 3 for light-water-cooled, gas-cooled, and liquid metal-cooled reactor systems. In the more recent revisions of Section XI, feedback from operating plants has resulted in new requirements to address stress corrosion cracking and other corrosion-related mechanisms, as well as weld overlay and other piping repair techniques, and also revisions to inspection and test requirements to address a number of other current issues such as nondestructive examination procedures used by nuclear power plant designers. The 2015 Edition includes substantial revisions. Some revisions were for clarifications based on the issued interpretations, and some were for improvements in the requirements based on industry experience. The changes included the incorporation of Code Cases N-532-5 and N-805, revision to Nonmandatory Appendix D, and the application of peening for enhancing surface properties. Finally, the chapter includes a discussion on some of the issues and efforts that did not result in the final action by the Committee, such as the rules for an inservice inspection of reactor coolant pump flywheels.