Mitochondria are dynamic organelles, often fusing and dividing in response to changes in the cellular environment, during cell division and differentiation and under many other circumstances that affect mitochondrial function. Mitochondrial division is also an integral part of the apoptotic pathway in mammalian cells. In recent years, significant advances have been made in the study of mechanisms underlying the mitochondrial division and fusion processes. A number of proteins that contribute to these processes were discovered. Prominent among these are three large GTP binding proteins, of which two are members of the dynamin family. Here, we review what is known about these proteins and discuss possible mechanisms for the mitochondrial fusion and division processes.