Microbially influenced corrosion is responsible for deterioration of concrete, sandstone, bricks, and plaster. Due to different metabolic activities of the bacteria involved, microbial sulfuric acid corrosion should be distinguished from microbial nitric acid corrosion. The former proved to be a reason for damage to sewage pipelines made of reinforced concrete. Deterioration proceeds above the sewage level. The latter proved to be involved in damaging overground buildings, esp. architectural monuments like the Cologne Cathedral. One group of wide-spread microorganisms are sulfur oxidizing bacteria of the genus Thiobacillus. They live from inorg. sulfur and sulfur compds. The other group called nitrifying organisms consists of ammonia and nitrite oxidizing bacteria. The substrate ammonia is mainly provided by dust deposition on stone surfaces. Nitrifiers of the genus Nitrobacter can oxidize nitric oxide to nitric acid. Thiobacilli and nitrifiers produce strong mineral acids, like sulfuric and nitric acid.