Parmelia sulcata, commonly known as the hammered shield lichen or cracked-shield lichen, is a foliose lichen in the family Parmeliaceae. First described by Thomas Taylor in 1836, it is one of the most prevalent lichen species globally, known for its resilience to pollution and cosmopolitan distribution across temperate and cold regions of both hemispheres. P. sulcata forms a circular thallus up to 10 cm (4 in) in diameter, with a glaucous white to grey upper surface and a black lower surface, featuring broadly lobed structures with both marginal and laminal soralia and a distinctive reticulate pattern of pseudocyphellae.

Taxonomically, Parmelia sulcata has a complex history. Molecular phylogenetics studies have revealed significant genetic variability within what was traditionally considered a single species, identifying a complex of cryptic species, including Parmelia encryptata and P. barrenoae, and demonstrate the necessity of molecular data for accurate species identification. P. sulcata maintains a highly specific symbiotic relationship with green algae of the genus Trebouxia, particularly species within Trebouxia clade I, as its photobiont partner.

Ecologically, Parmelia sulcata grows on various substrates, including bark, wood, and rocks. It accumulates pollutants such as heavy metals and radionuclides, reflecting air quality and contamination levels, making it a useful bioindicator in pollution studies across Europe, North America, and other regions. Beyond its ecological importance, P. sulcata has been used in traditional medicine and as a source of natural dyes.

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