Although molybdenum was first identified as an element over two centuries ago, its biological importance was not appreciated until researchers demonstrated it had a direct role in animal metabolism. Molybdenum has since been identified as an essential trace element for nearly all plants and animals, occurring as a cofactor in three important enzymatic reactions that take place in virtually all forms of life. Clinically, molybdenum deficiency is rare, but inborn errors of metabolism resulting in deficiencies of the molybdoenzymes have been described. Dietary intake of molybdenum is generally sufficient, with legumes such as lentils, beans, and peas being the richest source. Nuts, grains, cauliflower, and leafy vegetables are also good sources, whereas animal products and fruit are low in molybdenum. Molybdenum content of plant-based foods is dependent on the amount of molybdenum in the soil in which they are grown. Molybdenum supplementation may be of therapeutic benefit in patients with molybdo enzyme deficiency, sulfite sensitivity, Wilson’s disease, and certain types of cancer, and in those receiving total parenteral nutrition.