Mycotoxin (from Gk. μύκης (mykes) "fungus") is a toxin produced by a fungus under special conditions of moisture and temperature. These fungi are aerobic (use oxygen) and microscopic and, moreover, may colonise many kinds of food from the field to the table. Mycotoxins can appear in food and animal feed as a result of fungal infection of the crop, for example Fusarium ear diseases in cereals, or the infection of stored products. Not all fungi can produce mycotoxins. In addition some fungi are able to produce mycotoxins only in special conditions such as at specific levels of moisture, stress and the correct temperature. Even those with the ability to produce mycotoxins may not produce them all the time. The absence of mycotoxins doesn't ensure the absence of fungal spores, so it's possible for fungi to 'appear' when the temperature and humidity are right. In addition, the mycotoxins are very resistant to temperature treatments and to conventional food processes such as cooking, freezing etc.

Public concern over mycotoxins increased following some of the multi-million dollar "toxic mold" settlements of the late 1990s. The negative health effects of mycotoxins are a function of the concentration, duration of exposure and the subject's sensitivities. The concentrations experienced in a normal home, office or school are typically too low to trigger a health response in occupants. The mycotoxins are not gaseous (non-volatile), but rather they are associated with the spores and hyphae. Exposure typically occurs when contaminated food is eaten. In some cases this may include meat from animals that have themselves been fed contaminated feed.

In Europe, statutory levels of a range of mycotoxins permitted in food and animal feed are set by a range of European directives and Commission regulations.