Endolichenic fungi refer to microorganisms that reside within lichen thallus without causing any visible symptoms. These fungi share similarities with plant endophytes in terms of their taxonomic classification, diversity, mode of transmission, and evolutionary background. Although the fossil record provides evidence of a historical association between endolichens and fungi dating back to the Lower Devonian period, the current understanding of these fungi as lichen partners is considered to be relatively limited. The understanding of the biology of endolichenic fungi and their interactions with other components of the lichen microbiome remains limited. Nevertheless, several studies have underscored the significance of the secondary metabolites synthesized by these organisms and their potential implications for a diverse range of biological processes. The investigation of these facets of endolichenic fungi is imperative in order to enhance comprehension of their ecological dynamics and economic viability. In order to gain comprehensive insights into the involvement of endolichenic fungi in the establishment and perpetuation of lichen symbiosis, as well as their interactions with the microbiome of lichens, it is imperative to acquire a more profound comprehension of their biological characteristics. This chapter provides an analysis of the present comprehension regarding endolichenic fungi, encompassing their presence in various aspects, thereby emphasizing the necessity to reconsider the definition of lichen.