This Safety Guide provides recommendations and guidance on controlling activities relating to modifications to nuclear power plants so as to reduce risk and to ensure that the configuration of the plant is under control at all times, and that the modified configuration conforms to the approved basis for granting an operation licence. The recommendations cover the whole process from conception to completion for modifications to structures, systems and components, operational limits and conditions, procedures and software, and the management systems and tools for plant operation. This publication recommends how to meet the requirements established in IAEA Safety Standards Series No. NS-R-2, Safety of Nuclear Power Plants: Operation.