Molds are filamentous (fuzzy or dusty looking) fungithat occur in many feedstuffs including roughages andconcentrates. Molds can infect dairy cattle, especiallyduring stressful periods when they are immunesuppressed, causing a disease referred to as mycosis.Molds also produce poisons called mycotoxins that affectanimals when they consume mycotoxin contaminatedfeeds. This disorder is called mycotoxicosis. Mycotoxinsare produced by a wide range of different molds and areclassified as secondary metabolites, meaning that theirfunction is not essential to the mold’s existence. TheU.N.’s Food and Agriculture Organization (FAO) hasestimated that worldwide, about 25% of crops are affectedannually with mycotoxins (Jelinek, 1987). Such surveysreveal sufficiently high occurrences and concentrations ofmycotoxins to suggest that mycotoxins are a constantconcern. Tables 1 and 2 provide mycotoxin occurrenceand concentration of farmer submitted feedstuffs in NorthCarolina over several years.Mycotoxins can be formed on crops in the field, duringharvest, or during storage, processing, or feeding. Moldsare present throughout the environment. The spores arehigh in the soil and in plant debris and lie ready to infectthe growing plant in the field. Field diseases arecharacterized by yield loss, quality loss, and mycotoxincontamination. Mold growth and the production ofmycotoxins are usually associated with extremes inweather conditions leading to plant stress or hydration offeedstuffs, insect damage, poor storage practices, lowfeedstuff quality, and inadequate feeding conditions.It is generally accepted that the Aspergillus, Fusariumand Penicillium molds are among the most important inproducing mycotoxins detrimental to cattle. Themycotoxins of greatest concern include: aflatoxin, which isgenerally produced by Aspergillus mold; deoxynivalenol,zearalenone, T-2 toxin, and fumonisin, which are produced by Fusarium molds; and ochratoxin and PRtoxin produced by Penicillium molds. Several other mycotoxins such as the ergots are known to affect cattleand may be prevalent at times in certain feedstuffs.There are hundreds of different mycotoxins, which arediverse in their chemistry and effects on animals. It islikely that contaminated feeds will contain more than onemycotoxin. This paper is directed toward thosemycotoxins thought to occur most frequently atconcentrations toxic to dairy cattle. A more extensivereview is available in the popular press (Whitlow andHagler, 2004).