The occurrence of saprophytic moulds in indoor environments and their potential hazards to human health are discussed. In addition to mesophilic species, xerophilic moulds appear to be common, often developing together with mites. Allergic and non-allergic symptoms are reported when patients are exposed to moulds in homes, schools and working places. Air-conditioning systems are consistently involved with mould development. Complaints of eye-, nose- and throat-irritation as well as fatigue seem to be correlated with unpleasant odours produced by abundant mould growth, but the relationship between the symptoms and the odours is not understood. The role of air-borne mycotoxins is discussed. Methods to detect moulds in indoor environments are described. Because no single method or cultivation medium is sufficient to detect all the various indoor moulds, a combination of air sampling, direct microscopic examination and cultivation on both standard and low water activity media is recommended.