The first discovery of mycovirus was recorded in 1962. by the English virologist Hollings. In the last fifty years, interest in knowledge of mycoviruses is growing. To date, mycoviruses are known to be present in all taxonomic groups of fungi. The presence of mycovirus in the host may lead to a decrease in virulence and affect phenotype changes, but in some cases infections with mycoviruses are latent. The life cycle of mycoviruses takes place in the cyto-plasm of host cells. In order to spread successfully in cells, essential is vegetative compatibil-ity of host fungi. Recent literature reports possibility to bypass vegetative compatibility by protoplast fusion thus allowing the widening of mycoviruses host range. The presence of mycoviruses and their hypovirulent influence has been proven in laboratory conditions on economically important fungi belonging to the genus Fusarium, Botrytis, Rhizoctonia, Cryphonectria, Alternaria, etc. In Europe, phytopathogenic species Cryphonectria parasitica, the causative agent of chestnut blight disease, has been successfully controlled using Cryphonectria hypovirus 1 (CHV 1). In some species of pathogenic fungi coinfection of iso-lates with two or more mycoviruses has been reported. In addition to their inhibitory effect on virulence of pathogenic fungi, they can also have a beneficial effect on host plants by re-ducing the temperature stress. In order to be used as a agents for biological control of phy-topathogenic fungi additional research is needed, especially to better understand the inter-action of mycoviruses and their hosts.