Plant diseases caused by fungal pathogens are a major cause of yield loss in agriculture and forestry. Mycoviruses have been discovered in many important plant pathogenic fungi. The majority are RNA viruses, that cause persistent infections of their fungal hosts without having an extracellular phase. Virus infection occurs via hyphal anastomosis allowing the virus to move from a virus-infected to uninfected mycelium. While most virus infections remain asymptomatic, some are known to attenuate virulence and sporulation ability of their fungal hosts and thus can act as antivirulence elements against these pathogens. Biological control strategies against plant diseases have been developed using such mycoviruses, the best-known example being hypovirulence in the chestnut blight fungus, Cryphonectria parasitica. This chapter gives an overview about the basic concepts of mycovirus-mediated biological control of plant diseases and reviews examples of practical and potential applications of mycoviruses for disease control.