Concrete, stone, brick, plaster, wood, plastic, painted surfaces and metal are all colonised by bacteria, algae and fungi which accelerate theirdeterioration. The mechanisms of deterioration, the main microbial genera involved and factors which may affect the degree of colonisation and attack are discussed. The chief factor determining microbial growth on constructional materials is moisture. Thus it is important for architects and engineers to consider critical points in the humidity profile of a building at the design stage. Damp surfaces are readily colonised by microbial cells settling from the air. This leads to the formation of a biofilm, which can trap dust and other particulate materials, increasing its disfiguring effect. In addition, the biofilm can act as a reservoir for potentially dangerous microorganisms such as the bacteria responsible for legionnaires’ disease and allergenic fungal and actinomycete spores. Materials may be protected against microbial growth by the use of biocides. The use of such toxic agents is critically reviewed.