The most important ichnotaxa (and some biotaxa) of microendolithic cyanobacteria, algae and fungi are presented. Their relations to environmental factors such as light, temperature, salinity, inorganic nutrients and their bioerosion potential are discussed. From the Silurian on, they form characteristic ichnocoenoses that are bathymetrically controlled and can be used for the reconstruction of water depths of fossil marine basins. Some ichnotaxa appeared as far back as the Ordovician, and some biotaxa in the Precambrian.