Corrosion and scaling processes in thermal systems of buildings and in geothermal installations have been studied with a special focus on microbially induced corrosion and fouling. It was found that a certain series of microbial succession starting with autotrophic bacteria such as Gallionella spp. allows microbial life even in environments with very low nutrient- and organic carbon-content. Microbially initiated buildup of elemental sulfur and polysulfide is another key element of microbially induced corrosion. These patterns point towards a generalizable picture which is presented here. It is believed that this model, combined with recently developed knowledge about interactions with abiotic processes, will prove to be very helpful in further understanding of different phenomena observed in wells and pipework. A method was invented to monitor corrosion and scaling processes in these systems, based on pH, redox, oxygen partial pressure, temperature and conductivity.