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.

The genus Usnea was circumscribed by Michel Adanson in 1763. He used the name designated by Johann Jacob Dillenius, whose earlier published description did not meet the rules of valid publication as established by the International Code of Nomenclature for algae, fungi, and plants. Adanson did not specify a type specimen; the species Usnea florida, moved to the genus by Friedrich Heinrich Wiggers in 1780, has been designated as the lectotype.

Since the establishment of the genus, hundreds of Usnea species have been described. A three-volume series by Józef Motyka published between 1936 and 1947 listed 451 species. However, Motyka applied a strongly typological species concept, treating each minor morphological variant as a distinct species and resulting in an inflated taxonomy. Modern lichenologists recognize that Motyka's approach, which relied heavily on characteristics strongly influenced by environmental conditions (such as thallus color, branch thickness, presence of foveoles, and thallus length), has led to extensive synonymy and confusion within the genus. By 1998, more than 770 names had been published globally, and it was estimated that approximately half were synonyms.

Modern taxonomic treatments of Usnea emphasize a populational species concept, focusing on the variability within populations rather than fixed morphological ideals. This approach recognizes species based on correlated discontinuities in two or more independent characters, including chemistry, anatomy, and morphology, along with an absence or rarity of intermediate forms. Morphological characters such as the glossiness of the cortex, the pigmentation of the basal portion, the presence and morphology of soralia, isidiomorphs (structures resembling isidia but originating from medullary hyphae), and the form and arrangement of fibrils have proven particularly useful.

Chemical analysis, particularly through thin-layer chromatography, has become essential for accurately distinguishing species, as many morphological traits are influenced significantly by environmental factors. Philippe Clerc, a Swiss lichenologist who has specialized in studying Usnea, has cautioned against the recognition of species based solely on chemical differences ("chemotypes") unless strongly correlated morphological or anatomical differences are also present.

By the late 1990s, modern interpretations based on the populational concept, extensive field and herbarium studies, and routine chemical analyses were contributing to a substantial reduction in the total number of recognized species, as previously named taxa were increasingly recognized as synonyms or environmentally induced variations of fewer, more broadly defined species. Clerc estimated at the time that around half of the more than 770 published names would eventually be reduced to synonymy, while also noting that the genus remained incompletely known, especially in tropical regions. Historically, the inflated number of species resulted partly from Motyka's limited opportunities for extensive field studies outside Europe and his reliance on typological rather than populational concepts. Motyka's limited access to advanced chemical analytical tools and his emphasis on characters easily observable with limited optical equipment also contributed to the historical complexity and confusion in Usnea taxonomy.

Molecular studies have highlighted the importance of accurate species identification in the genus Usnea, demonstrating that incorrect identifications can significantly distort phylogenetic analyses. For instance, careful morphological and anatomical re-examinations of voucher specimens previously used in molecular studies revealed that supposed synonymies, such as those between U. barbata and U. dasopoga, were incorrect. Instead, these species are distinct and separable by anatomical measurements and chemistry, underscoring the necessity of integrating careful morphological analysis with molecular techniques in systematic studies. Recent molecular barcoding studies of Usnea have shown mixed results. While internal transcribed spacer (ITS) barcoding provides reliable identification for some species, it fails to fully resolve complex aggregate taxa, such as the U. cornuta complex. Identification through simple BLAST searches of public databases can be problematic due to misidentifications in published sequences. Evidence suggests some species groups within Usnea are undergoing rapid evolution, with traditional species circumscriptions sometimes containing multiple distinct genetic clades or being intermixed within individual clades. This evolutionary complexity creates additional challenges for taxonomists working to establish stable species boundaries within the genus.