Optical analyses for colour, transparency and reflectance of all organic fossils may be used for palaeotemperature data. Of the organic-walled fossils, acritarchs, chitinozoans, dinoflagellate cysts, graptolites, pollen and spores are of particular value in geothermometry. At increasing temperatures, fossil organic material shows progressive changes in colouration and increasing reflectance. Palaeotemperatures of sedimentary rocks may be derived from the optical characteristics of the main organic fossil groups recorded. Regional palaeotemperature values are associated with variations in overburden thickness and heat flow. Regional tectonic provinces typically record a restricted range of palaeotemperature values that may be related to sediment thickness and phases of igneous and tectonic activity.