Lichens represent one of biology's most celebrated examples of mutualistic symbiosis, existing as composite organisms where the fungal and algal/cyanobacterial partners are so intimately integrated that lichens were historically classified as single organisms before their dual nature was recognised by Simon Schwendener in 1869. Today approximately 20,000 lichen species are known, classified primarily by the identity of their fungal partner. The composite nature of lichens results in properties neither partner possesses alone: lichens can colonise the most extreme environments on Earth including bare rock faces, polar regions, high mountain peaks, and desert surfaces where temperatures may range from -40°C to +70°C, UV radiation is intense, and water availability is intermittent. Lichens are considered ecosystem engineers because they are among the most important pioneer organisms in primary succession — they colonise bare rock, produce acids (oxalic acid, lichen acids) that slowly dissolve minerals from rock surfaces, and contribute organic matter as they die, initiating the process of soil formation that eventually allows mosses, then other plants, to establish.

The relationship between fungal and algal partners in lichens is primarily mutualistic, with both organisms benefiting: Fungal partner (mycobiont): provides the physical structure of the lichen thallus (almost entirely composed of fungal hyphae), protects the algal partner from desiccation (by forming a dense outer cortex) and UV radiation, absorbs water from rain and dew, and concentrates mineral nutrients from the substrate. Algal/cyanobacterial partner (phycobiont): photosynthesises to produce organic carbon (primarily glucose and polyols like ribitol and erythritol that are specifically released to the fungus by the algal cells) that feeds the fungal partner; in lichens containing cyanobacteria (particularly Nostoc species), the cyanobacterial partner also fixes atmospheric nitrogen (N2 → NH4+), providing a nitrogen source for the lichen in nitrogen-poor environments. However, the relationship is not entirely equal — the fungal partner has greater structural control and some researchers argue it verges on controlled parasitism (the fungus extracts nutrients from the algal cells in a manner that the algae might not freely provide if not constrained within the fungal structure), making lichen symbiosis one of the most debated examples regarding the "mutualism vs controlled parasitism" spectrum in biology.

It is important to clearly distinguish among the different types of fungal symbioses, as these are frequently confused in examinations: Lichen = fungus + alga (or cyanobacterium): a terrestrial composite organism with its own name and thallus; the alga is embedded within the fungal structure. Mycorrhiza = fungus + plant root: both partners maintain their separate identities (the fungus does not engulf the root); the fungus colonises the root surface (ectomycorrhiza) or penetrates root cells (endomycorrhiza/arbuscular mycorrhiza); affects mineral absorption of the plant. Endophytes: fungi living within plant tissues (inside leaves, stems, roots) without causing obvious harm; some provide protection against herbivores or pathogens. Nitrogen-fixing root nodules: bacteria (Rhizobium in legumes, Frankia in alder etc.) form nodules in roots — this is a bacterium-plant association, not fungal. Coralloid roots: cyanobacteria (Nostoc, Anabaena) harboured in specialised lobed roots of Cycas — plant-cyanobacterium association.

Lichens perform numerous important ecological and economic roles. Pioneer colonisers: as discussed, lichens are the first organisms to colonise bare rock, initiating primary succession and soil formation. Nitrogen fixation: lichens with cyanobacterial phycobionts (particularly Lobaria and Peltigera species in temperate forests) can contribute significant amounts of fixed nitrogen to forest ecosystems. Pollution indicators: lichens are highly sensitive to air pollution, particularly sulfur dioxide (SO2) and nitrogen compounds. Their absence or reduced diversity in urban and industrial areas serves as a bioindicator of air quality — "lichen deserts" around polluted industrial sites are used in air quality monitoring. Food source: Cladonia rangiferina (reindeer lichen/reindeer moss) is the primary winter food source for reindeer and caribou in arctic and subarctic ecosystems, illustrating their importance as base food web components in extreme environments. Dyes: historically, several lichens yielded important textile dyes — Ochrolechia tartarea gave purple litmus dye (also the source of the acid-base indicator litmus), and Rocella tinctoria gave orchil dye. Traditional medicine: Usnea species produce usnic acid with antibiotic properties used in some preparations.