Individual mitochondria which form the chondriom of eucaryotic cells are highly dynamic systems capable of fusion and fragmentation. These two processes do not exclude one another and can occur concurrently. However, fragmentation and fusion of mitochondria regularly alternate in the cell cycle of some unicellular and multicellular organisms. Mitochondrial shapes are also described which are interpreted as intermediates of their "equational" division, or fission. Unlike the fragmentation, the division of mitochondria, especially synchronous division, is also accompanied by segregation of mitochondrial genomes and production of specific "dumbbell-shaped" intermediates. This review considers molecular components and possible mechanisms of fusion, fragmentation, and fission of mitochondria, and the biological significance of these processes is discussed.