This chapter gives an overview on (1) lichen-forming fungi, lichen photobionts and peculiarities of lichen symbiosis such as gains and losses of lichenization, species concepts, specificity, morphodemes and morphotype pairs, non-lichen mutualistic fungal interactions with unicellular algae and cyanobacteria and mycophycobioses; (2) the mycobiont–photobiont interface, water relations and gas exchange, mycobiont-derived secondary metabolites and the accumulation of heavy metals or radionuclides; (3) the microbiome of lichen thalli, i.e. the bacteriome (epi- and endolichenic bacteria), lichenicolous and endolichenic fungi, lichenicolous lichens and the virome of lichens and their allies; (4) fossil lichens and their microbiome; (5) lichen–animal interactions such as the micro- and mesofauna of lichen thalli, lichenivory in invertebrates and vertebrates, endo-and epizoochory; (6) lichenomimesis in animals and flowering plants