Mycoviruses are viruses that infect fungi. The majority of mycoviruses have double-stranded RNA (dsRNA) genomes and isometric particles but approximately 30% have positive-sense, single-stranded RNA (+ssRNA) genomes. True mycoviruses demonstrate an ability to be transmitted to infect other healthy fungi. Many double-stranded RNA elements that have been described in fungi do not fit this description and in these cases, they are referred to as virus-like particles or VLPs. Preliminary results indicate that most mycoviruses co-diverge with their hosts, i.e. their phylogeny is largely congruent with that of their primary hosts. However, many virus families containing mycoviruses have only sparsely been sampled. Mycovirology is the study of mycoviruses. It is a special subdivision of virology and seeks to understand and describe the taxonomy, host range, origin and evolution, transmission and movement of mycoviruses, and their impact on host phenotype.