Xanthoria parietina, the golden shield lichen, is a conspicuous yellow foliose lichen. It is found in sunny, exposed places that are rich in nitrogen. Many species of lichen-forming fungi contain yellow or orange extracellular pigments belonging to the dibenzofurans (usnic acid), anthraquinones (e.g. parietin) or pulvinic acid group. These pigments are all equally efficient light screens, leading us to question the potential ecological and evolutionary significance of diversity in yellow and orange lichen substances. Xanthoria parietina produces an orange colored anthraquinone pigment, parietin, that gets deposited as tiny crystals in the top layer of the upper cortex. As a widespread lichen, it is coloured by the orange cortical pigment parietin. This pigment protects the photosynthetic apparatus of Xanthoria parietina against damage by high light levels. The accumulation of the protective pigment parietin in Xanthoria parietina is induced specifically by UV-B radiation.