This chapter presents the specific aspects of epidemiology, disease associations and public health significance of vitamin B12 deficiency in the developing countries. In East Asia, data is mostly available from China where vitamin B12 deficiency is prevalent in all age groups, though little data is available from pregnant women. The chapter highlights the importance of vitamin B12 nutrition across the lifecycle starting with genetic factors, and from pregnancy to infancy, childhood, adolescence, adulthood and elderly. An important realization is that, poor maternal vitamin status during pregnancy and lactation translates into a depleted storage in the child. Vitamin B12 deficiency is a major public health challenge in pregnant and lactating women from developing countries. There are only a few vitamin B12 supplementation trials in developing countries. Vegetarians have a substantial difficulty in achieving adequate daily intakes, and supplementation or food fortification need to be considered. B-complex supplementation trials in the western non-vegetarian populations have failed to reduce the Cardio Vascular Disease.