Distributed at all latitudes, lichens are able to conquer extreme biotopes, adapting to living in places where plants would find it difficult to survive: they overcome critical periods by rapidly going into dormant phases during which respiration and photosynthesis are reduced. When environmental conditions become favourable again, they very quickly absorb enough water to resume photosynthesis and carry out all other metabolic activities. They are able to colonise all kinds of natural and man-made substrates: soil, rock, tree bark, wood, leaves of evergreen plants, but also concrete, bricks, tiles, glass, asbestos, etc. The use of lichens by humans dates back to the ancient Egyptians, who used some species to make bread flour. Today they are used as food, decoration, in the preparation of dyes and perfumes, toextract antibiotics and antimycotics and to conduct scientific investigations of various kinds. They are used, for example, to date substrates of unknown age, such as glacial moraines and archaeological finds; they give information about the substrate in relation to pH, moisture content, presence of nitrogenous substances, etc.; they enable detection of air pollutants.