Vitamin K is receiving more attention in relation to its role in bone metabolism. Vitamin K is a coenzyme for glutamate carboxylase, which mediates the conversion of glutamate to gamma-carboxyglutamate (Gla). The gamma-carboxylation of the Gla proteins is essential for the proteins to attract Ca2+ and to incorporate these into hydroxyapatite crystals. The best known of the three known bone-related Gla proteins is osteocalcin (OC). Even though the exact role of OC is not known, a number of studies have shown that vitamin K insufficiency or high levels of undercarboxylated osteocalcin (ucOC) is associated with an increase in the concentration of circulating ucOC. Furthermore, several studies have demonstrated that vitamin K insufficiency is associated with low bone mineral density (BMD) and increased fractures. Vitamin K supplementation, on the other hand, has been shown to improve the bone turnover profile and decrease the level of circulating ucOC. Dietary recommendations are based on saturation of the coagulation system, and in most countries the dietary intake is sufficient to obtain the amount recommended. In relation to bone, requirements might be higher. The aim of this chapter is to give an overview of the importance of vitamin K in relation to bone health in adult humans and thereby in the prevention of osteoporosis. Furthermore, I will shortly discuss the interaction with vitamin D and the paradox in relation to warfarin treatment.