Effects of molds and mycotoxins on health and performance of ruminants are often difficult to distinguish from each other. Molds may adversely affect ruminant production by any or all of the following: reducing nutritive value of feed, limiting feed intake, affecting physiological function directly; or indirectly by mycotoxicoses. Molds associated with mycotoxin production are: Aspergillus sp. (aflatoxin); Penicillium sp. (penicillic acid and patulin); and Fusarium sp. (deoxynivalenol and zearaienone). Wet and cool weather conditions favor Fusarium sp. growth and hot and dry conditions favor Aspergillus sp. growth. Ruminants and chickens are quite tolerant to deoxynivalenol, apparently due to gastrointestinal microbial activity: Deoxynivalenol concentrations (diet DM) of up to 8.5 ppm in lactating dairy cow diets, 21 ppm in growing cattle diets, or 15.6 ppm in growing sheep diets had no adverse effects on health or production performance. No deoxynivalenol residue was found in milk of cows fed 6.4 ppm (diet DM) for 70 d. Ewes and heifers are sensitive to zearalenone effects on reproduction when concentration exceeds 0.5 or 5 ppm (diet DM), respectively. Current literature on effects of ochratoxin or T-2 toxin on performance is limited; therefore, no obvious trends were evident to suggest tolerance levels of these toxins. A simplified action chart has been devised to suggest management alternatives when molds or mycotoxins are suspected.