Lichens are the outcome of a sustained mutualism between a fungal and a photosynthesizing partner (alga or cyanobacterium). Lichens are associated with endolichenic fungi that reside within their thalli, in addition to the fungal companion in this mutualism. The endolichenic fungi seem to have evolved in tandem with the lichen, and a significant number of them are a source of novel metabolites that possess distinctive bioactivities. The biology of endolichenic fungi and their interactions with the other components of a lichen microbiome are extremely limited. It is imperative to comprehend these aspects of endolichenic fungi in order to gain a more comprehensive understanding of their economic potential and ecology. This review summarizes the current state of understanding regarding endolichenic fungi.