Usnea is a genus of fruticose lichens in the large family Parmeliaceae. The genus, which currently contains roughly 130 species, was established by Michel Adanson in 1763. Species in the genus grow like leafless mini-shrubs or tassels anchored on bark or twigs. Members of the genus are commonly called old man's beard, beard lichen, or beard moss. Usnea lichens are characterized by their shrubby growth form, elastic branches with a central cord, and distinctive soralia that produce vegetative propagules. They vary in colour from pale green to yellow-green, grey-green, reddish, or variegated, and range in size from a few millimetres in polluted areas to over three metres long in species like Usnea longissima.

Members of the genus are similar to those of the genus Alectoria. A distinguishing test is that the branches of Usnea are somewhat elastic, but the branches of Alectoria snap cleanly off. Usnea species are widely distributed across temperate and tropical regions worldwide, growing primarily on trees but occasionally on rocks, and are sensitive bioindicators of air quality, thriving only in unpolluted environments. The genus has a complex taxonomic history, with many species historically over-described due to environmental variations, though modern approaches using chemical, morphological, and molecular analyses have helped clarify relationships. Usnea lichens have been used traditionally for medicinal purposes, textile dyes, fire starters, and occasionally as emergency food, and serve important ecological roles including providing nesting material for birds.

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