Abstract Molybdenum (CAS No. 7439-98-7) is a silvery-white transition metal and an essential micronutrient for most life forms. It is rarely found in its elemental form but rather in combination with sulfur, oxygen, or calcium or with another metal. It enters the environment from anthropomorphic activities such as coal combustion, mining, and smelting and can be found in industrial waste facilities. Molybdenum is found in leafy vegetables, legumes, meat, and many grains. Generally, molybdenum is of low toxicity to both mammals and fish with the exception of domestic ruminants, which are the most sensitive to molybdenum exposure. The liver appears to be the target organ.