From the epidemiological studies conducted on the effect of mould and dampness on health a decade ago, the role of toxin-producing fungi in damp and mouldy buildings involved opinion more than evidence. Very little was known about the metabolites that were produced by the fungi that grew on damp building materials, and almost nothing had been reported on their occurrence in buildings. As a consequence, the focus was on speculations involving the fungal toxins that occur in agriculture. Over the past decade, particularly in the last 5 years, considerable progress has been made concerning the relevant toxins from fungi that grow on damp building materials. This paper summarizes the available data on the low-molecular-weight toxins reliably known from fungi common on damp building materials, the toxins that have been measured on mouldy building materials and the new understanding of the role that they play in the documented health effects of individuals living and working in damp and mouldy buildings.