‘Fungal mutualisms’ considers symbiotic relationships in which the fungus and its partner benefit from their biological interaction. Examples of mutualisms with insects include fungi that trap scale insects, fungi cultivated by ambrosia beetles, and leaf-cutter ants and termites that grow mushroom gardens. These highly developed relationships involve substantial structural, biochemical, and behavioural adaptations in the fungi and insects. Fungi in mycorrhizal associations with plants operate as accessory root systems for plants, whereas fungi called endophytes house themselves inside plant tissues without any connection to an external mycelium. Lichens—composite organisms produced by a fungus and a single-celled alga or cyanobacterium—are the best-known mutualisms involving fungi.