Mycotoxins are toxic metabolites produced by common fungi. The most important in the world’s food and feed supplies are: aflatoxins, ochratoxin A, fumonisins, deoxynivalenol, and zearalenone. Aflatoxins, produced by Aspergillus flavus and related species in grains and nuts, cause liver cancer, often in association with hepatitis B virus, and some other effects. Ochratoxin A is produced by both Aspergillus and Penicillium species in various foods including grains (in cold climates), coffee, dried grapes, and wines. It causes kidney damage and is also carcinogenic. Fumonisins are produced by Fusarium verticillioides and related species in maize. They cause brain liquefaction in horses, liver cancer in rats, and perhaps esophageal cancer in humans. Deoxynivalenol is the most commonly occurring of a large group of mycotoxins called trichothecenes, which are toxic because they interfere with protein synthesis. Deoxynivalenol and zearalenone, which is an estrogenic mycotoxin, are produced by Fusarium graminearum and related species in small grains, particularly wheat. This chapter provides an overview of the sources, occurrence, toxicity, and control of the major mycotoxins.