In the vast majority of eukaryotic somatic cells the nucleolus is the largest and the most dynamic nuclear domain. The canonical nucleolar function is ribosome biogenesis, which includes ribosomal DNA (rDNA) transcription, pre-rRNA processing and ribosomal subunit assembly. Furthermore, according to recent data the nucleolus and specific nucleolar proteins participate in cell cycle regulation, apoptosis and control of aging. These functions are mostly realized by the nucleolus at interphase and become downregulated during mitosis, when the nucleolus disassembles. This review summarizes the current data on the dynamics and mechanisms of the disassembly and reassembly of the nucleolus during mitosis. Particular attention is given to the information obtained by analysis of the dynamics of the nucleolus in living cells and by modeling of the premature assembly of the nucleolus under various experimental conditions. © Pleiades Publishing, Ltd. 2007.