Hemophilia A (factor VIII deficiency) and Hemophilia B (factor IX deficiency) are the most common and serious congenital coagulation disorders. Accurate diagnosis is important and essential for effective management. A definitive diagnosis depends on factor assay to demonstrate the presence of factor VIII or factor IX. Bleeding should be treated with factor replacement therapy at the earliest moment possible, preferably within two hours from the onset of symptoms. In spite of improvements in hemophilia therapy, arthropathy remains as a significant clinical problem. Based on numerous recommendations, a major goal of hemophilia therapy is to prevent any joint disease, and prophylaxis is superior to on-demand therapy in delaying or preventing the development of hemophilic arthropathy. Prophylaxis is the administration of clotting factors at regular intervals to prevent bleeding. Currently the most commonly suggested protocol for prophylaxis is the infusion of 25~40 IU/kg of clotting factor concentrates three times a week for those with hemophilia A and twice a week for those with hemophilia B. The management of patients who have inhibitory antibodies against factor VIII or IX remains challenging. About 10~15% of hemophilia A patients and 1~3% of hemophilia B patients may develop persistent inhibitors rendering treatments with factor concentrates difficult. Alternative agents for hemophilia inhibitor patients include bypassing agents, such as recom-binant factor VIIa and prothrombin complex concentrates. Ultimately, immune tolerance induction to eradicate the inhibitor is desired.