Micronucleus (MN) is a small additional nucleus and is readily identifiable by light microscopy. Biologically, micronuclei are the chromosome fragments or whole chromosomes that lag behind at anaphase during nuclear division. MN occurs due to genetic damage of the cell and the MN scoring is the indicator of the genetic damage. Therefore MN scoring can be used in various clinical setting such as to supervise genotoxicity, biomonitoring of diseases, screening of preneoplastic diseases and identification of high risk patients. In this brief review, the morphology, etiology, estimation, and applications of MN have been discussed. Diagn. Cytopathol. 2012. © 2010 Wiley Periodicals, Inc.