Deficient activity of an enzyme can result from a defect in the conversion of the vitamin to a co-enzyme as well from an abnormal apo-enzyme or disturbed binding of coenzyme to enzyme. Conversion of dietary vitamin to intracellular active co-enzyme can be complex and require many physiological and biochemical processes including stomach release of bound vitamin, intestinal uptake, carriers/transport, blood transport, cellular uptake, intracellular release and intracellular compartmentalisation. Disorders of malabsorption (food cobalamin malabsorption, intrinsic factor deficiency and abnormal enterocyte cobalamin processing) and transport proteins (transcobalamin II deficiency, R-binder deficiency) mostly lead to disturbed function of the two cobalamin requiring enzymes, methylmalonyl CoA mutase and methionine synthase. Defects of early steps of intracellular cobalamin (cblF, cbl C/D) result in marked deficiencies of both cobalamin co-enzymes and homocystinuria combined with methylmalonic aciduria. Defective synthesis of adenosyl cobalamin in the cbl A/B defects leads to methylmalonyl CoA mutase. Isolated methionine synthase deficiency is also classified as a cobalamin disorder due to its associated deficient formation of methylcobalamin. Folate disorders include methylene-tetrahydrofolate reductase deficiency and glutamate formimino-transferase deficiency. In addition a hereditary disorder of intestinal folate transport has been described. Less well established are disorders of dihydrofolate reductase, methenyl-tetrahydrofolate cyclohydrolase, and defects of cellular folate uptake.