Although micronuclei (MNi) have been extensively used to evaluate genotoxic effects and chromosome instability, the most basic issue regarding their formation was not completely addressed until recently, due to limitations of traditional experimental methods. The development of live-cell imaging, combined with genetically engineered chromosome labelling techniques makes it possible to investigate the origin of a micronucleus in a single cell in a real-time and high-throughput manner. Here, we review all the available studies on the origins of MNi in live cells and discuss novel findings based on this recently emerged methodology. Some unsolved questions on MNi formation and limitations of live-cell imaging in the investigation of MNi have also been discussed.