The Class Arthoniomycetes is a group of fungi within the phylum Ascomycota, primarily known for forming lichens, though some species are free-living or parasitic. Members of this class are mostly lichenized, meaning they live in symbiosis with algae or cyanobacteria, producing a thallus that can vary in form from crust-like to leaf-like. They are characterized by their bitunicate asci—sac-like structures that produce sexual spores—and often have distinctive fruiting bodies called apothecia or lirellae. Arthoniomycetes are ecologically important as bioindicators, contributing to nutrient cycling in ecosystems, and are widely distributed, especially in tropical and temperate regions. Their diversity and adaptations make them a significant group for studying fungal evolution and lichen ecology.