Moulds represent the biggest proportion in the number of approximately 250,000-300,000 species which belong to the kingdom fungi. Their role in nature and their different stages of multiplication on appropriate substrates are described. The conditions for multiplication of toxin-producing moulds are not principally different for non-toxogenic species. Most important for the multiplication of moulds are the following parameters: temperature, substrate composition, amount of light, gas compound; they are discussed. The quantitative results of mycological laboratory investigations of feed are given in the number of colony forming units (c.f.u.) per gram and can be interpreted in three categories: normal content (mostly not perished feed), increased content (mostly less fresh) and excessively high content (mostly perished). Often the results of the cultural investigation do not correspond with the investigation on mycotoxins. The reasons for that are mentioned.