Abstract Bioerosion of carbonate substrates by microorganisms more or less constitutes a subdiscipline of its own. This distinction from macroscopic bioerosion is due to the fact that the organisms involved are dominantly plants, fungi and prokaryotes. Because the borings themselves are so small, they demand special techniques for their study. Consequently, microscopic bioerosion is a field for specialists that is rarely entered by workers in related fields. Several animal groups bioerode on a microscopic scale. Radula traces produced by chitons and snails can barely be seen with the naked eye. The borings produced by phoronids and bryozoans also are so small that they can only be studied using the same casting techniques that are used for microscopic bioerosion. This chapter deals with endolithic plants, fungi and prokaryotes, which, although taxonomically diverse, form an ecologically homogeneous group. The organisms involved are listed in Table XI-1. The chiorophytes comprise a number of genera and species that actively bore in carbonate substrates. They produce branching networks of tunnels varying greatly in size. Some may be as little as 2μ m in diameter. In some species such fine filaments may collect together in main canals as wide as 25 μ m, but including sack-like chambers of greater size (Golubic, Perkins and Lukas, 1975, p. 243). A shell or light-colored rock that has been thoroughly bored by green algae takes on a pale green color. Two genera of rhodophytes, Porphyra and Bangia , are endolithic in early stages of their life cycle, after which they become epilithic.