 Bark-Biedels and fungi form mutually beneficial relationships known as ectosymbiotic associations. These associations vary widely in terms of the level of dependency, the mode of transmission, and the environmental factors involved. They also differ in terms of the number of partners involved and the degree of fidelity and longevity. Evolutionary processes, including phylogeny, abiotic environment, and interactions within the biotic community, have played key roles in shaping these associations. This article was authored by Diana L. Six.