Cultural heritage monuments may be discolored and degraded by growth and activity of living organisms. Microorganisms form biofilms on surfaces of stone, with resulting aesthetic and structural damage. The organisms involved are bacteria (including actinomycetes and cyanobacteria), fungi, archaea, algae, and lichens. Interactions between these organisms and stone can enhance or retard the overall rate of degradation. Microorganisms within the stone structure (endoliths) also cause damage. They grow in cracks and pores and may bore into rocks. True endoliths, present within the rock, have been detected in calcareous and some siliceous stone monuments and are predominantly bacterial. The taxonomic groups differ from those found epilithically at the same sites. The nature of the stone substrate and the environmental conditions influence the extent of biofilm colonization and the biodeterioration processes. A critical review of work on microbial biofilms on buildings of historic interest, including recent innovations resulting from molecular biology, is presented and microbial activities causing degradation are discussed.