Hyphomycetous fungi – so called ‘mould’– are the most important agents of biodeterioration in museums, museums' storage rooms, in libraries, collections and restoration studios. Fungi are able to live at low water activities, they are perfectly adapted to indoor environments and thrive in microclimatic niches caused by condensation, lack of ventilation or water retention by hygroscopic materials. Fungi spoil valuable pieces of art aesthetically, mechanically, chemically and by degradation of organic components. Historical material made of paper and oil paintings with high amounts of organic binders are especially susceptible to fungal deterioration. In order to prevent fungal contamination or to treat already contaminated objects an integrated approach including climate control, material-specific cleaning and application of carefully selected biocides is necessary.