Fungi are distributed worldwide, as they can grow on a wide range of substrates and under various conditions. Fungi not only play an important role in most ecosystems, but they have a direct impact on society due to production of secondary metabolites. From a big array of fungal secondary metabolites, some compounds are beneficial for the society (e.g. antibiotics), and others (e.g. mycotoxins) represent a threat to human and animal health and cause economic losses.

This chapter provides an overview of fungal secondary metabolites with emphasis on one of the most important groups, mycotoxins. The different classes of these metabolites are discussed. Due to an enormous diversity in chemical structures, biosynthetic origins and biological effects, only exemplary classifications are provided, which, however, form a complete and clear picture of similarities and differences between the metabolites. Furthermore, their impact on human and animal health is described. The different routes of human and animal exposure (ingestion, inhalation, dermal route) to mycotoxins are also presented and supported by occurrence data.