Harmful microorganisms in indoor environments pose a threat to human health and damage building materials. Ms. Messal provides comprehensive and well-founded expertise on mold infestations in indoor spaces. She explains all the steps involved in creating a damage assessment report, from professional sample collection to microbial diagnostics, the relevant chemical-physical testing methods, and the interpretation of findings. In cases of mold damage, bacteria, mites, and protozoa are usually also involved. The assessment of the extent of damage further depends on the requirements for indoor hygiene, which have been newly defined in the mold guideline by the Federal Environment Agency. The author discusses the health aspects of visible and hidden damage patterns and critically examines the new UBA guideline, addressing legal aspects as well.