This manual is a revision of Technical Reports Series No. 260, Biological Dosimetry: Chromosomal Aberation Analysis for Dose Assessment (1986). It provides the latest information on standardized conventional methods used for the cytogenetic assessment of doses incurred through ionizing radiation (scoring dicentric chromosomes) and on newly available proven techniques such as fluorescence in situ hybridization, premature chromosomal condensation and micronucleus assays. Contents: 1. Introduction; 2. Application of dose concepts in biological dosimetry; 3. Biophysical background to chromosome damage; 4. Human lymphocytes; 5. Chromosomal structure; 6. Radiation induced chromosomal alterations; 7. Blood sampling; 8. Dicentric analysis; 9. Translocation analysis; 10. Premature chromosome condensation (PCC) analysis; 11. The Micronucleus (MN) assay; 12. Automatic analysis of chromosomal alterations; 13. Safety of laboratory staff; References; Annexes — working protocols: Annex I: Dicentric assay; Annex II: FISH based translocation assay; Annex III: Premature chromosome condensation assay; Annex IV: Micronucleus assay.