This chapter summarizes the diversity of interactions in lichen symbiosis at the taxonomic, organismic and cellular levels. Peculiarities of lichen-dominated terrestrial ecosystems, their ecological roles and endangerment are discussed. Lichenicolous (parasitic) fungi, symptomless endophytic fungi and bacterial epibionts of lichen thalli and their potential biological roles are summarized. The focus is on the symbiotic phenotype of lichen-forming ascomycetes, i.e. the main types of thallus morphologies, the functional anatomy of macrolichens and their growth patterns, the fine structure and composition of cell wall types of lichen photobionts and their impact on the mycobiont–photobiont interface. Modes of vegetative symbiotic propagation are summarized.