The paper summarizes the current state of knowledge regarding the role of filamentous microorganisms (i.e., fungi and actinomycetes) and their submicrometer propagules (fragments) in formation of indoor bioaerosol. It discusses the importance of water damages in buildings and the role of humidity as a cause of fungal and actinomycetal contamination and subsequent deterioration of indoor spaces. The importance of the size of airborne microbial propagules for adverse health effects is broadly commented as well. Regarding the microbial fragments, the method of their release from the contaminated surfaces (including factors influencing their aerosolization, i.e., air velocity, colony structure, moisture conditions, vibration of the surface, time factor), modern measurement techniques and newly obtained results of the immunological reactivity of fragments are discussed. The novel ideas concerning the dynamic description of the release process of microbial propagules from their sources are also presented.