This document discusses the phylogeny and evolution of lichens. It notes that lichens are formed through a symbiotic relationship between algae and fungi, so studying their evolution requires examining both partners. There are approximately 20,000 known lichen species, though they are not considered biological species. Current research focuses on determining how many times lichen symbiosis has evolved among fungi. Lichens can take several morphological growth forms. Their taxonomy is complex with lichen-forming fungi found in many orders of the ascomycota. Phylogenetic analyses provide evidence that lichen symbioses have evolved multiple times and many fungal lineages originated from lichen ancestors.