Mitoviruses, classified within the Mitoviridae family of the Lenarviricota phylum, are among the most elementary RNA viruses known. They consist of naked virions lacking a capsid envelope and harbor positive single-stranded RNA (ssRNA) genomes with a low Guanine Cytosine (GC) content. These genomes typically feature a single open reading frame encoding an RNA-dependent RNA polymerase (Widner et al. Mol Cell Biol 11:2905–2908, 1991). Notably, mitoviruses replicate solely within the mitochondria of plants, fungi, and invertebrates. Translation within mitochondria employs the UGA codon for tryptophan, a departure from its conventional role as an “opal” stop codon. Additionally, mitoviral sequences, serving as non-retroviral endogenous RNA viral elements, are prevalent in numerous plant nuclear and mitochondrial genomes. Horizontal transmission of mitoviruses primarily occurs through hyphal anastomosis and heterokaryosis, while vertical transmission takes place via spores. Despite often being cryptic, these viruses can influence fungal growth and contribute to virulence in plants. This chapter aims to delve into various aspects of mitoviruses, including their taxonomy, transmission dynamics, and molecular characteristics.