Vitamin B12 (B12, cobalamin (Cbl)) is a water-soluble vitamin that requires complex mechanisms for its assimilation, blood transport and intracellular metabolism. Three proteins, intrinsic factor (IF), haptocorrin (HC), and transcobalamin (TC), and their specific receptors are involved in B12 absorption and transport. Acquired and inherited deficiencies can result in megaloblastic anemia and neurological manifestations. Several genetic diseases are linked to these two steps, namely inherited deficits in FI and TC, and Imerslund-Gräsbeck disease. In mammalian cells, only two enzymes depend on vitamin B12: L-methylmalonyl-CoA mutase (EC 5.4.99.2) in mitochondria, and methionine synthase (EC 2.1.1.13) in cytoplasm. Direct metabolic consequences of impaired B12 absorption and metabolism are the accumulation of methylmalonic acid (MMA) and of homocysteine (HCy), respectively. More than a dozen genes are involved in the intracellular metabolism of B12, and their defects result in several diseases designated cblA through cblJ This article reviews the steps involved in vitamin B12 absorption, transport and intracellular metabolism, and the main related genetic defects.