The cytokinesis-block micronucleus assay in peripheral blood lymphocytes is a standardised and validated technique for biodosimetry. Automated scoring of micronuclei allows large scale applications as in population triage in case of radiation accidents or malevolent use of radioactive sources. The dose detection limit (95% confidence) of the micronucleus assay for individual dose assessment is restricted to 0.2 Gy but can be decreased to 0.1 Gy by scoring centromeres in micronuclei using fluorescence in situ hybridization (FISH). In the past the micronucleus assay was applied for a number of large scale biomonitoring studies of nuclear power plant workers and hospital workers. Baseline micronucleus frequencies depend strongly on age and gender. The assay was also already used for biodosimetry of radiation accidents. In a multiple endpoint biodosimetry study for dose assessment of a worker exposed accidentally in 2003 to X-rays, a good agreement was obtained between dose estimates resulting from the micronucleus assay, the scoring of dicentrics and translocations. Automated scoring of micronuclei in combination with centromere signals, allowing systematic biodosimetry of exposed populations, remains a challenge for the future.