Lichenized fungi initiate their symbiotic structures from microscopic stages after recog-nition of compatible algae. The partnerships ultimately emerge as complex macroscopic phenotypes which are unrivaled in the fungal kingdom by their resilience and durability. This chapter presents an overview of lichen symbioses and covers the morphology and systematics of the fungal phenotypes, as well as their associations with diverse photobionts. This is followed by a coarse overview of eco-physiology and the secondary chemistry. A special focus is given to the diversity of and theinteractionswithadditional microorganisms. Finally, a few comments on the effects of pollution and environmental change point to the usefulness of lichens as bioindicators. Keywords: Symbioses · Morphology · Mycobiont · Photobiont · Microbiome.