Mycotoxins are abundant secondary metabolites of mould fungi. Ochratoxins were first detected in Aspergillus ochraceus but are produced by other Aspergillus and Penicillium species as well. Ochratoxin A is frequently found in human blood where it is distributed after intestinal absorption. It is the most abundant mycotoxin in natural food and food products at ppb (parts per billion) level. In man the toxin is slowly eliminated by kidney and liver and is supposed to cause kidney diseases but also liver and urinary tract tumors. The concern about this toxin has promoted the release of consumer protection regulations in many countries including the European Union. Maximal residue levels for specific food items have been set. The toxicological background of these developments is briefly described.