Vitamin K (phylloquinone, K1; menaquinone, K2) functions as an essential cofactor for the synthesis of the coagulation protein factors II, VII, IX, X and protein C and S by promoting a unique post-translational modification of specific glutamic acid residues to gamma-carboxylglutamic acid, thus mediating calcium binding to phospholipid surfaces. Vitamin K deficiency results in a depletion of liver stores of phylloquinone, decreased plasma levels of vitamin K1, increased levels of K1 epoxide, appearance of noncarboxylated protein (PIVKA), decreased levels of functioning vitamin K-dependent clotting factors and prolongation of the APTT, PT and thrombotest. When the progression of deficiency leads to abnormal clotting tests a generalized bleeding tendency occurs. Noncarboxylated prothrombin (PIVKA-II) determinations are a sensitive indicator of vitamin K deficiency. Although Vitamin K deficiency can occur at any age (warfarin, fasting, antibiotic therapy, malabsorption syndromes) the major public health problem is related to prevention of early, classic and late hemorrhagic disease of the newborn (HDN). A single dose of oral or parenteral vitamin K prevents classic HDN but the most effective way to prevent early HDN is by giving large doses to the mother prior to delivery (2 weeks). Late HDN in breastfed infant occurs with a prevalence of about 20 per 100,000 live births when no neonatal prophylaxis is given. Parenteral (1 mg) K1 prevents late HDN and single or repeated doses of oral vitamin K reduces the incidence but does not eliminate all late HDN. The optimal (cost, feasibility, effective) mode of neonatal prophylaxis remains to be determined.