Vitamin B12 is unique among all the vitamins in that it contains not only a complex organic molecule but also an essential trace element, cobalt. Vitamin B12 is synthesized in some bacteria but not in animals and plants. Intestinal absorption and subsequent plasma transport of vitamin B12 are mediated by specific vitamin B12-binding proteins and their receptors in mammals. Vitamin B12 taken up by the cells is enzymatically converted to coenzyme forms of vitamin B12, methyl- and adenosyl-vitamin B12, which function as coenzymes of methionine synthase (EC 2.1.13), involved in methionine biosynthesis, and methylmalonyl-CoA mutase (EC 5.4.99.2), involved in oxidation of odd-numbered fatty acids and amino acids (valine, isoleucine, and threonine), respectively. Chemical properties, physiological function, and intracellular metabolism of vitamin B12 are summarized in this section.