Like other lichens it is a symbiosis of two or three fungi and an alga. In Usnea, the fungus belongs to the division Ascomycota, while the alga is a member of the division Chlorophyta.
Many species have been described. Many of these are now regarded as morphological varieties and adaptations to local circumstances. The taxonomic categorization of many members of this genus remains uncertain.
Species in this genus have an elastic chord or axis running through the middle of the thallus that can be revealed by gently pulling a filament apart from either end. Based on a fossil found in Baltic amber, it is known that the genus dates back to at least the late Eocene, about 34 million years ago. The elastic central cord that keeps the branch of the lichen intact even when its cortex is split is an adaptation that has helped allow the genus to thrive in many ecosystems and climates.