This report details the results of the Water Reactor Fuel Extended Burnup Study (WREBUS), the first internationally conducted study of its kind. The report assesses the economic effects of burnup extension, evaluates uranium utilization for PWRs and BWRs (e.g. the need for higher enrichments and possible fuel design changes) and considers environmental, safety and licensing implications. The results of the study provide a basis for understanding incentives and indicating directions for future developments in fuel cycle technologies and for introducing improvements in fuel performance and operational strategies. Contents: 1. Introduction; 2. Incentives for extending burnup; 3. Economic evaluation; 4. Technical aspects; 5. Conclusions; Appendix: Description of the fuel cycle cost method used for the US contribution.