Ruminants have the capacity to utilize some mycotoxin contaminated feedstuffs without impact on production or carry-over tissue residues. Despite large investments in crop development to diminish mold invasion and mycotoxin production, grain facilities to dry and store cereals, and use of alternative processing, mycotoxins frequently occur at elevated concentrations that affect ruminants. Fungal invasion by molds can occur in stored forages, silages, and wet bales and toxicity of these mold related mycotoxins is often poorly characterized. Ruminants occupy wide agricultural niches that expose animals to diverse toxins in different conditions, challenging veterinarians making diagnostic interpretations on contaminated forages and grains. This article discusses mycotoxins affecting ruminants in North America.