Lichens are formed by a fungus (usually an ascomycete) and a photosynthetic partner, which is usually a green algae (Chlorophyta) but in some cases may be a cyanobacteria.

Over 800 lichens have been found in Iceland to date and several new species are discovered every year.

In Iceland, lichen-forming fungi belong to a total of 17 different orders. The majority (just over 500 species) belong to order Lecanorales. Around 70 species belong to order Peltigerales, and another 70, or so, species belong to order Verrucariales. Fewer than 50 species belonging to order Pertusariales are found in Iceland. Other orders are represented by 1–17 species. In addition to this, at least one Basidiomycota fungus species in Iceland forms lichen.