Article IWA-4000 of Section XI of the Boiler and Pressure Vessel (B&PV) Code contains the requirements for performing repair/replacement activities on nuclear power plant items. The repair/replacement activity requirements of IWA-4000 apply to a component after it has met all of the Construction Code requirements. This chapter examines the background of these repair/replacement activities, notes some of the significant restructuring of paragraphs and terminology and important changes in repair/replacement activity requirements since the original 1970 Edition, and reviews and discusses the requirements in IWA-4000 in the 2015 Edition of Section XI. This chapter is beneficial to personnel performing such repair/replacement activities as designing plant modifications, obtaining replacement items, and performing welding, brazing, defect removal, fabrication, installation, examination, and pressure-testing activities.