Mycotoxins are mold produced feed contaminants that although undesirable are mostly unavoidable consequences of feed production. The level of mycotoxins in foods and feed can fluctuate widely and vary significantly from year to year. Although several hundred mycotoxins have been described in the scientific literature, less than 10 have been extensively studied since the discovery of aflatoxin in the early 1960s. Ruminant diets include both forages and concentrate grains, which may increase the risk of mycotoxins in comparison with grain-based monogastric diets. The multiplicity of ingredients in complex diets may increase the probability of multiple mycotoxin contamination but decrease the risk of high mycotoxin concentrations. It has been widely accepted that ruminants, due to microbial degradation, are less susceptible to mycotoxin poisoning than non-ruminants. However, rumen metabolites of the parent mycotoxin may be equally or more toxic. Because of this, differences between parent mycotoxins and their metabolites are much greater in ruminants than non-ruminants. Mycotoxins can increase disease incidence and reduce production efficiency in ruminants. They can cause dermal toxicity, reproductive effects, carcinogenicity, neurotoxicity, teratogenicity, nephrotoxicity, and hepatoxicity. Additionally, mycotoxins may affect immune function and cause lipid peroxidation. In spite of current research advances, applied aspects of mycotoxicology are either limiting or difficult to extrapolate into the real world. This review will attempt to discuss some of the most common problems related to presence of mycotoxin in ruminant diets.