Fumonisins are toxic metabolites of the fungus Fusarium moniliforme, which is a common contaminant of corn everywhere in the world. The fumonisins are carcinogenic in laboratory rats, and cause acute toxicity of domestic animals that mimics field cases of disease attributed to contamination of feed by F. moniliforme. These include both equine leukoencephalomalacia and porcine pulmonary edema. Fusarium moniliforme contamination of corn consumed by humans in certain areas of the world is associated with higher than average incidence of esophageal cancer, and fumonisins may be responsible. Analytical methods have been developed for fumonisins, but improvements are needed so that more accurate, less expensive, and more rapid assays of food and feedstuffs can be done. Fumonisins are structurally similar to sphingosine, and may exert their biological activity through their ability to block key enzymes (sphinganine‐ and sphingosine‐N‐acyltransferases) involved in sphingolipid biosynthesis. Much more research is needed to define the extent to which this myco‐toxin adversely affects the food supply, and its involvement in animal and human diseases.