Endolichenic fungi (ELF) reside inside the lichen thalli without showing any visible symptoms of their presence. The fossil record of endolichenic fungi dates back to 415 million years; however, they have been successfully isolated only since the 1990s. Approximately, 500 endolichenic fungi have been isolated of which only 135 have been identified up to species level. A large number of endolichenic fungi are either partially identified up to genus level or are treated as unidentified. Among them, only 114 well-known species of lichens belonging to 22 families have been utilized for isolation of endolichenic fungi, while in some cases hosts are not identified. These lichens mostly belong to macro lichens, i.e., members of family Parmeliaceae being dominant. Occurrence of the endolichenic fungi on different groups of plants indicates that they are not host-specific. Endolichenic fungi live in close association with algal photobionts and share many similarities in terms of taxonomy, diversity, transmission mode, and evolutionary history with fungal endophytes of plants. The networks of endophytic fungi are less nested, less connected, and more modular than endolichenic networks. The diversity of ELF community is largely affected by the size of thalli fragments. Endolichenic species generally do not show host specificity. The role of ELF in lichen symbiosis has not yet been fully described; however, some hypotheses have been put forward. To gain deeper understanding of biology of endolichenic fungi, it is essential to explore their role in establishing and sustaining the lichen symbiosis and their interactions within lichen microbiome. Moreover, comprehensive studies of endolichenic fungi require employment of molecular methods.