Cliostomum corrugatum is a crustose lichen in the family Ramalinaceae, notable for producing characteristically wrinkled or corrugated apothecia that distinguish it from related taxa. The genus Cliostomum is a small group of saxicolous and corticolous crustose lichens with a disjunct distribution in temperate and boreal regions of the Northern Hemisphere, including Europe and North America. The thallus of Cliostomum corrugatum is thin and pale, often nearly immersed in the bark substrate on which it grows, while the pale tan to cream apothecia are flat to slightly convex with the distinctive corrugated or rugose surface. The species is found on the bark of deciduous trees in old, undisturbed woodland, including veteran oaks, beeches, and other broadleaved trees with stable, relatively unpolluted bark chemistry. Cliostomum species are considered indicators of ecological continuity in woodland habitats and are included in lists of ancient woodland indicator species in northwestern Europe. The species is sensitive to air pollution, particularly sulfur dioxide and nitrogen compounds from agricultural and industrial sources, and declined significantly across industrialized regions of Britain and Europe during the 20th century. Recovery has been noted in areas where air quality has improved. The species is of conservation concern in several European countries.