This report describes and assesses radiological characterization as a precursor to decommissioning. It shows the influence of the radioactive inventory on the planning and strategies of decommissioning and also presents an extensive overview of characterization results on various reactors which have been or are being decommissioned. Contents: 1. Introduction; 2. Characterization objectives; 3. Health and safety considerations; 4. Characterization process; 5. The radiological inventory; 6. Methods and techniques for characterization; 7. Quality assurance requirements; 8. Conclusions; Appendix: Statistics for radiological characterization; Annexes I–1 to I–9: National experience in various Member States; Annex II: Problems encountered in the characterization of nuclear reactors and lessons learned.