A post-implementation review (PIR) is a structured evaluation after a change has been deployed (e.g., a new system, migration, or security patch). The review checks whether the change achieved its objectives, whether unexpected side effects occurred, and what lessons can be drawn for future changes. PIRs are a core part of the change management process under ISO 27001 Annex A.8.32. You should define the review date in the change request itself, typically a few days to weeks after implementation. The results feed into your lessons-learned register.