The fundamental objective of the Incident Reporting System (IRS) is to contribute to improving the safety of commercial nuclear power plants which are operated worldwide. This fourth publication, covering the period 2005-2008, follows on the success of the previous three. It highlights important lessons learned based on a review of the approximately 200 event reports received from the participating countries over this period. The book is written both for a non-technical audience and for senior officials in industry and government who have decision making roles in the nuclear power industry.