Parmelia is a genus of medium to large foliose (leafy) lichens. It has a global distribution, extending from the Arctic to the Antarctic continent but concentrated in temperate regions. There are about 40 species in Parmelia. In recent decades, the once large genus Parmelia has been divided into a number of smaller genera according to thallus morphology and phylogenetic relatedness.

It is a foliaceous lichen, resembling a leaf in shape. The ends of the leaf-like lobes are often squarish-tipped. The upper surface is pale bluish-gray to light brown in direct sunlight, with a network web-like ridges and depressions. The lower surface is black and has rhizines anchoring it to the substrate. In general, Parmelia have a dark lower side with rhizines ('rootlets') that attach the lichen to its substrate. The upper side may be several colours - grey, yellow, brown - and may have reproductive organs on it. These may be apothecia (ascospore-producing bodies), isidia or soralia (both vegetative structures). In between these two layers is the medulla, which contains the algal component of the lichen.

Parmelia was circumscribed by Swedish lichenologist Erik Acharius in 1803. His idea of the genus, which included foliose species with lecanorine apothecia, was quite broad and included species that are now dispersed in many different genera and families, such as the Peltigeraceae (Lobaria), the Pannariaceae (Pannaria, Parmeliella), the Physciaceae (Physcia, Heterodermia, Physconia), the Teloschistaceae (Xanthoria), as well as the Parmeliaceae (Cetraria, Hypogymnia, and Parmeliopsis). Its broad circumscription meant that almost every lichen with a thalline apothecial rim was included by 19th-century authors.

In an attempt to create more homogeneous groups of taxa, lichenologists created new segregate genera for species once in Parmelia. These included Menegazzia (1854), Parmotrema (1860), Anzia (1861), Parmeliopsis (1869), Hypogymnia (1896), Pseudevernia (1903), Pannoparmelia (1912), and Pseudoparmelia (1914). In the 1970s and 1980s, electron microscopy was used to help divide several Parmelia species groups into different genera based on the structure of their cortex. These include Arctoparmelia, Bulbothrix, Canoparmelia, Cetrariastrum, Concamerella, Everniastrum, Flavoparmelia, Hypotrachyna, Neofuscelia, Paraparmelia, Parmelina, Parmotrema, Pseudoparmelia, Relicina, Relicinopsis, Xanthomaculina, and Xanthoparmelia. Another group of species was segregated on the basis of the presence of pseudocyphellae: Punctelia, Flavopunctelia, and Melanelia. In Mason Hale's 1987 monograph on Parmelia, he commented: "The group has been further subdivided ... now leaving in Parmelia a small, apparently irreducible assemblage of species typified by P. saxatilis". In 2016, however, sixteen mostly Australasian species were moved to the new genus Notoparmelia; these species had been shown by molecular phylogenetic analysis to form a monophyletic lineage in Parmelia. Targeted sampling in western North America also uncovered two species within the P. saxatilis complex: P. imbricaria (a rock-dwelling taxon with copiously overlapping inner lobules and predominantly marginal pseudocyphellae) and P. sulymae (a humid-forest species with narrow lobes and marginal, soredia-like propagules). Both were described from British Columbia and adjacent regions on the basis of combined morphology and multilocus phylogeny.

Traditional morphological and chemical characteristics have proven insufficient for reliable species identification within Parmelia. For example, while pruina (a dusty looking surface appearance) was previously considered a diagnostic feature in isidiate Parmelia species, studies have shown that individual specimens can vary in their degree of pruinosity. Similarly, in Parmelia species without vegetative propagules, the presence of lobaric acid was once considered diagnostic. However, this secondary metabolite has been found to be inconsistently present within specimens of the same species. The incorporation of molecular methods in Parmelia taxonomy has led to the discovery of cryptic taxa, such as P. encryptata and near-cryptic species like P. rojoi.

Most diversification within Parmelia appears to have taken place during the Neogene, with subsequent radiations in the Pleistocene. A time-calibrated multilocus phylogeny dated the crown group of Parmelia (in the strict sense) to the early Miocene (about 19.5 Ma), with the main lineages corresponding to the P. saxatilis and P. sulcata groups diverging roughly 10 Ma. Several species lineages then split or diversified during later episodes of global cooling and habitat reorganisation, including Pliocene splits (e.g., P. sulymae–P. serrana) and Pleistocene radiations within complexes such as P. saxatilis and P. encryptata. These timing patterns are consistent with climate-driven range shifts and fragmentation through the Miocene–Pleistocene.

There are two foliose fossil taxa, Parmelia ambra and P. isidiiveteris, that have been placed provisionally in genus Parmelia due to their overall resemblance to members of this genus. Later authors have suggested, however, that this generic placement is not appropriate for the current concept of Parmelia, and that because of the dearth of specimens available for analysis, it is impossible to know for certain which of the many foliose genera in the family Parmeliaceae is best suited for these fossils.

Parmelia species have a foliose (leafy) thallus with a substrate attachment ranging from loose to tight. The lobes comprising the thallus are rounded, more or less straight, and may be contiguous or overlapping (imbricate). The texture of the upper thallus ranges from smooth to foveolate (covered with puts and depressions). The colour is typically green to whitish grey to greyish brown, and some species have a coating of pruina on the surface. Most species have pseudocyphellae (tiny pores that allow for gas exchange), and vegetative propagules such as isidia or soredia, or both. The lower surface of the thallus is black (or close to it), and has rhizines (either simple or branched) that function as holdfasts to attach it to its substrate. The cortex (botany) is paraplectenchymatous – a cell arrangement where the hyphae are oriented in all directions.