Fungi are ubiquitous inhabitants of rock and mineral surfaces and are significant geoactive agents. Capable of numerous transformations of metals and minerals, fungi can prosper in the most adverse of environments, their activities underpinned by growth form and metabolism. Free-living filamentous species, microcolonial fungi and lichens can significantly change a rock's surficial structure and appearance, ranging from discolouration and staining to biodeterioration and the formation of new biogenic minerals and rock coatings. The presence and activity of fungi should be considered in any study of rock and mineral transformations that seeks to understand the biotic and abiotic processes that underpin geochemical change in the biosphere.</p