Almost all buildings experience excessive moisture, leaks, or flooding at some point. If dampness-related problems are to be prevented, it is essential to understand their causes. From a technologic viewpoint, one must understand the sources and transport of moisture in buildings, which depend on the design, operation, maintenance, and use of buildings in relation to external environmental conditions such as climate, soil properties, and topography. From a societal viewpoint, it is necessary to understand how construction, operation, and maintenance practices may lead to dampness problems. The interactions among moisture, materials, and environmental conditions in and outside a building determine whether the building may become a source of potentially harmful dampness-related microbial and chemical exposures.