In general, microorganisms such as chemolithotrophic and chemoorganotrophic bacteria, cyanohacteria, algae, fungi, and lichens living on and in stone material may be of importance in biodeterioration. These groups contribute substantially to the deterioration of mineral materials such as natural stone, concrete, ceramics, and glass. The attack on mineral materials may be either strongly or mildly aggressive or both. A strongly aggressive attack is caused by biogenic mineral or organic acids. A mildly aggressive attack results from hydrophilic slimes such as heteropolysaccharides and/or proteins (biofilm) and their ability to accumulate water and salts. Attack by exoenzymes seems to be unimportant. In recent years it has become possible to test the resistance of mineral materials to microbial attack by means of a biotest. Three simulation apparatuses were constructed; each allowed the incubation of test materials under microbiologically optimized conditions. Biodeterioration involving biogenic sulfuric acid corrosion, which under natural conditions needs eight times as long, was detectable within a few months. The results differed from those of purely chemical and/or physical tests of materials. Physical/chemical test methods are not sufficient to determine the resistance of materials to biological attack, because they do not include the interactions between microorganisms and their substrate, the mineral material. In the case of biogenic sulfuric acid corrosion, simulation experiments demonstrated differences in resistance of various concrete types, which ranged from I to 20% weight loss of test blocks within I year. With chemical testing only negligible differences in weight loss were noted. This was also the case with biogenic nitric acid corrosion. Thus, biotests assist in the selection of appropriate materials from many different ones.