The nucleolus is the largest and most dynamic nuclear domain in the vast majority of eukaryotic cells. The main and universal nucleolar function is participation in ribosome biogenesis, including ribosomal DNA (rDNA) transcription, pre-rRNA processing and ribosome subunit assembly. Furthermore, the nucleolus and its proteins also participate in cell cycle regulation, apoptosis and cell aging. These nucleolar functions are realized predominantly in interphase and, apparently, are abolished during mitosis, when the nucleolus disassembles. In this review, literature and our own data on the dynamics and mechanisms of the nucleolus disassembly and reassembly during mitosis in animal and plant cells are summarized. Particular attention is paid to the results obtained by analysis of the nucleolar dynamics in living cells and to modeling of the premature assembly of nucleolus upon various experimental conditions. The work was supported by the Russian Foundation for Basic Researches (grant 06-04-49392) and program RAS «Molecular and Cell Biology».