Mold consists of microscopic fungi that develop and feed on organic materials. Many of the materials that make up paintings on wood panel and canvas offer the perfect conditions for the development of these organisms. Animal glues, wood, starch, vegetable fibres and skins are the materials that are most subject to this type of risk. An environmental relative humidity that is over 60-70%, scarce ambient light, temperature above 15 °C and the presence of stagnant air are the conditions that favour the diffusion of microorganisms. In addition, the fact that a painted surface is not flat favours the adherence of spores that can cause spotting, colour alterations and changes in the mechanical characteristics.