Micronuclei (MN) are small, extranuclear bodies that arise in dividing cells from acentric chromosome/chromatid fragments or whole chromosomes/chromatids lagging behind in anaphase and are not included in the daughter nuclei at telophase. The mechanisms of MN formation are well understood; their possible post-mitotic fate is less evident. The MN assay allows detection of both aneugens and clastogens, shows simplicity of scoring, is widely applicable in different cell types, is internationally validated, has potential for automation and is predictive for cancer. The cytokinesis-block micronucleus assay (CBMN) has been widely applied in human biomonitoring studies of populations exposed to occupational/environmental mutagens. Furthermore, it allows assessment of nucleoplasmic bridges, nuclear buds, cell division inhibition, necrosis and apoptosis, and in combination with FISH using centromeric probes, the mechanistic origin of the MN. Therefore the CBMN test can be considered as a “cytome” assay covering chromosome instability, mitotic dysfunction, cell proliferation and cell death. In this chapter we aim to review the mechanistic basis and fate of MN as a biomarker of early genetic effects. In addition the different methodologies, automation of MN scoring and applications of the MN assay in human biomonitoring are highlighted.