This chapter describes the general nutritional aspects of cobalamin including content in food, dietary sources, bioavailability, forms of the vitamin in food, and dietary requirements. Several cobalamin- dependent enzymes are present in algae; but no species of plants has been found to have the enzymes needed for synthesis of the vitamin. Humans depend on cobalamin provided by consumption of animal-based foods such as eggs, milk products, meats, fish, seafood, and poultry. Based on radioactivity-based methods in humans and animals, the bioavailability of cobalamin is considered high from milk and chicken and comparable to that of free crystallized cobalamin dissolved in water. The chapter presents the biochemical aspects of cobalamin including its role as a methyl donor and discusses in relation, acquired cobalamin deficiency. Cobalamin deficiency also causes low methionine and therefore, low S-adenosylmethionine, the main methyl donor in the cell. The acquired cobalamin deficiency is caused by nutritional factors and by malabsorption.