The genus is cosmopolitan, and has been extensively studied in various regions in the past several decades, with significant biodiversity in South America identified as a central diversity hotspot.

The main characteristics that separate Physcia from similar genera in the same order, including Dirinaria, Heterodermia, Hyperphyscia, Kashiwadia, Phaeophyscia, and Pyxine, are the distinct morphology of its ascospores (brown and two-celled), its somewhat cylindrical pycnoconidia (asexual reproductive structures), and the presence of the chemical atranorin in the upper cortex.

Physcia has been divided into sections based on morphological and chemical characters, such as the presence or absence of cilia on the thallus margins and K+ (yellow) spot test reaction in the cortex.

Infections by these fungi can cause distinct physical symptoms useful for identification, such as the gall formations by Syzygospora physciacearum and the orange discolouration by Marchandiomyces auranticus.

Additionally, the long cilia of Physcia adscendens, which confer velcro-like attachment capabilities to the thallus of this species, are used by birds in nest building.

[11] The Swedish lichenologist Roland Moberg has authored or co-authored numerous scientific publications on the taxonomy and phytogeography of Physcia and related genera.

One source suggests that this name alludes to the lichen's apothecia and soredia found on its foliose thallus, which give the surface a vesiculose ("blistered") aspect when these reproductive structures develop.

[13] In an alternative etymological interpretation, John Alan Elix suggests that the name was originally intended to refer to species with inflated or hollow lobes.

However, advanced molecular methods have challenged the traditional concept of species pairs within the genus Physcia, which was previously defined largely through these similarities.

These findings indicate that distinctions based on morphological and chemical characteristics alone do not necessarily reflect the true genetic diversity and evolutionary relationships within the genus.

[17] The thallus of Physcia is foliose, meaning it is leaf-like in structure, and lobate, typically forming a roughly circular shape with lobes radiating outward.

[20] The surface can be matt or slightly glossy and might display minute white spots known as pseudocyphellae (tiny pores that allow gas exchange) visible with magnification.

[18] Although the value of pruina as a taxonomic character is sometimes considered dubious due to possible environmental influences, a group of species has a constant, fine, and homogeneous pruinosity, mainly on the distal parts of the upper thallus surface: P. atrostriata, P. krogiae, P. phaeocarpa, P. undulata, and P. verrucosa.

[18] In contrast, the lower surface of Physcia atrostriata is distinctively prosoplectenchymatous, featuring a brown-black colour with striations, which sets it apart from other species in the genus.

[18] Physcia species can be distinguished and grouped into distinct taxa based on the position and shape of their soralia, which provides valuable characteristics for identification and classification.

Following detailed examination of various Physcia species through electron microscopy, Mason Hale discerned that the atypical lower cortex comprises two distinct layers.

[24] The chemical profile of Physcia mediterranea, a rare Mediterranean species, has been studied with advanced phytochemical techniques, revealing a complex mixture of bioactive compounds, including the discovery of a kaurene-skeleton diterpene, previously undetected in lichens.

A reliable characteristic of Physcia is the consistent presence of atranorin in the upper cortex, unlike Hyperphyscia where it is rarely present, and Phaeophyscia where it is absent.

The genus Kashiwadia is distinguished from Physcia by several key features: it has significantly narrower thalline lobes and both its upper and lower cortical layers are paraplectenchymatous.

[29] Species of the genus Physcia grow on bark, wood, and rock, often in environments rich in nutrients or those that have been enriched by human activity.

[30] Similarly, Physcia tenella, a nitrophilous lichen, is reestablishing itself in its former habitats around Toruń, Poland, due to improvements in air quality resulting from reduced industrial emissions.

For example, infection by Syzygospora physciacearum results in the formations of galls, whereas others can be recognised by their discolouration: Marchandiomyces auranticus is orange, Illosporiopsis christiansenii is pink, while parasitism by Bryostigma epiphyscium creates tiny black spots.

[30] The species Physcia adscendens is characterised by its long cilia, which have been observed to function akin to cladding on the exteriors of bird nests.

Utilising characteristics derived from studies in Europe and North America, traits and taxonomy of Physcia species can be effectively adapted for monitoring nitrogen levels in tropical ecosystems, thereby supporting environmental management and conservation initiatives.

This prevalence is attributed to its tolerance to pollution, where unlike more sensitive species, Physcia does not die off but thrives, thereby serving as a reliable bioindicator for monitoring long-term air quality in urban settings.

[113] This process is most efficient under specific conditions and naturally occurs in a way that spontaneously releases heat, suggesting it could be a cost-effective and eco-friendly option for or the removal of metal pollution.