Deterioration of stone used in buildings and works of sculptural art proceeds by the same mechanisms responsible for natural weathering and soil formation. Algae usually appear first and, although corrosive, the usual objection is that they mar the appearance of facades through discoloration. The presence of algae leads to infestation by lichens and other plants. Growth of algae depends on the presence of several nutrients, especially calcium and magnesium, thus explaining the preference of algae for calcareous walls, rocks, and plaster. Lichens, a temporary symbiotic relationship of aerophytic algae and fungi, may cause serious damage by growing in mass and enlarging fissures, changing in volume during wet and dry periods, accumulating water on the stone surface, and by excreting organic acids, most commonly oxalic acid. Various measures for removing and controlling growth of algae and lichens are discussed.