In buildings growing conditions for mould fungi can occur and cause fungus infestation. Thepossible danger for the occupants of dwellings lies in the production and spreading ofpathogens (disease causing agents). Therefore, consequent measures have to be taken to avoidhealth dangers that result from mould fungi in buildings. A strategy has to be set up thatfocuses on the growth conditions for mould fungi and also considers the complex transientprocesses of building physics. The boundary conditions for the growth of fungi aretemperature, humidity and substrate conditions which have to be simultaneously favourableover a certain period of time. In the Fraunhofer-Institute of Building Physics (IBP) inGermany a biohygrothermal procedure is developed, which allows the prediction of mouldgrowth under transient boundary conditions. In order to differentiate the mould fungiaccording to the health dangers they may cause, so-called hazardous classes were defined.This new method will be described with its new features and its limitations. Typical results ofthis model are shown within an example. Furthermore the significance of the results isdiscussed.