Moulds grow wherever it is moist and warm. Mould consists of a cell network that constantly releases microscopic particles, so-called spores, which are actually the seeds of the mould fungi. These fulfil all properties to act as an allergy trigger: they can occur in high concentrations in the air, be inhaled, and trigger immune reactions. In Central Europe, about 10 types of mould are recognised as significant allergens. Moulds in the living area prefer poorly ventilated, damp rooms (bathroom, WC), but can also be found growing on damp walls, wallpaper, wood cladding, mattresses, humidifiers, garbage cans, and flower pots. Unfortunately, mould cannot always be detected with the naked eye. Mouldy foods can also release large amounts of mould spores. Moulds are also found outdoors, namely in compost, leaves, and grass. Therefore, mildew spores (especially in late summer/autumn) are floating in the air outside and can cause similar symptoms to those of hay fever. The most important allergy-causing mould fungi are called Alternaria and Cladosporium. Spores of the species Penicillium, Mucor, Aspergillus, and even Botrytis, the “noble rot” of grapes, can also cause allergies. Allergies to the spores of mould fungi can be distinguished from infectious diseases by an existing infestation of the fungi in the living space. The diagnosis of mould spore allergy relies on a corresponding medical history and further investigative steps such as skin test and laboratory examination.