This paper presents a study performed by the European Clearinghouse on Operational Experience for nuclear power plants, in cooperation with IRSN and GRS and covering events related to modifications of nuclear power plants. This study summarizes the analysis of more than 700 modifications related events reported to different databases. To help identify generic lessons learned and recommendations, the events were classified into categories reflecting the stages of the modification process such as design, manufacturing and supply of components, implementation of the modifications, post-modification testing and secondly, into sub-categories related to components: civil engineering, electrical components and instrumentation & control, etc. This paper presents the main findings and recommendations raised by this study. © 2013 Elsevier B.V.