Vitamin K is required for the hepatic production of blood coagulation factors II, VII, IX and X. The term vitamin K refers to a variety of fat-soluble 2-methyl-1, 4-naphthoquinone derivatives. Vitamin K1 (phylloquinone) occurs in green plants, while vitamin K2 (menaquinone) is synthesized by microbial in the gut. The recommended dose of vitamin K in neonates is 1 mg intramuscularly. For infants below 2.5 kg, the dose is 0.3 mg/kg to a maximum dose of 1 mg. Vitamin K crosses the placenta poorly, and neonates are relatively deficient of vitamin K at birth. Human milk contains about 1.5 µg/l of vitamin K, whereas most formula milks contain about three times as much as this. Vitamin K1 prophylaxis with 0.2 mg administered intramuscularly maintains adequate vitamin K1 status of preterm infants until a median age of 25 postnatal days and did not cause early vitamin K1 2, 3-epoxide accumulation. There is need for intramuscularly vitamin K prophylaxis for all newborns in order to eradicate hemorrhagic disease of the newborn. Some authors found that the antenatal administration of vitamin K1 to pregnant women in preterm delivery increases the blood coagulation activity in neonates whereas other authors found that the antennal administration of vitamin K1 has no effects on neonates. Vitamin K1 crosses the placenta poorly and it is not surprising that vitamin K1 has limited effects on neonates. The aim of this study was to review the effects of vitamins K in neonates and young infants.