Venous thromboembolism is predominantly a disease of older age. The average annual incidence rate of venous thromboembolism among Caucasians is 1 per 1,000 person-years. Common independent VTE risk factors include hospitalization, active cancer, neurological disease with extremity paresis and, among women, oral contraceptives, pregnancy and the puerperium, and hormone therapy. Inherited reductions in plasma natural anticoagulants (antithrombin, protein C, or protein S) have long been recognized as uncommon but potent risk factors for venous thromboembolism. Activated protein C resistance (factor V Leiden), G20210 variant in the prothrombin gene have been recently added to the list of inherited disorders predisposing to thrombosis. Those latter frequent inherited factors interact with environmental risk factors such as oral contraceptives, pregnancy, hormone therapy.