Hemophilia is the most common coagulation disorder. It has a long history. Hemophilia A is caused by FVIII gene mutation, and hemophilia B by FIX gene mutation. Those genes are located on X chromosome long arm. Bleedings in hemophiliacs predominantly occur in joints and muscles. Because those site are insufficient in tissue factor to induce hemostasis. Among joints knee, ankle and elbow are most frequently affected because their synovial structure is vulnerable to injury compared to other joints. Hemophilia is diagnosed with factor assay. Severe hemophilia is below 1% of FVIII : C, moderate between 1% and 5%, mild over 5%. Carrier detection and prenatal diagnosis have been conducted with RFLP-based linkage analysis and DNA sequencing. Mainstay of treatment is factor replacement therapy so far. Bleedings can be controlled by infusion of factor concentrates. Hemophilc arthropathy and muscle contracture are representative sequelae. Complications of facotor replacement therapy are inhibitor development and infections. Hemophiliacs with inhibitor should be managed with large dose factor concentrate, bypassing agent, ITI and immunosuppression. Ultimately, hemophilia could be cured by gene therapy.