Within the last few years, the knowledge of hereditary and acquired risk factors for venous thromboembolism has increased. Antithrombin-, protein C- and protein S-deficiency have been known since a long time as hereditary risk factors. Since 1993, three hitherto unknown risk factors have been described, the APC (activated protein C) resistance, hyperhomocysteinemia and a polymorphism in the 3-UT region of the prothrombin gene. These risk factors are relatively common in the normal population (in total 10-15%) and are found in 30-50% of patients with venous thromboembolism. The most important acquired risk factor for thromboembolism is the antiphospholipid antibody syndrome (APLS). The APLS is found in around 3% of patients with thromboembolism, patients with these abnormalities have a high risk for recurrency. The upper mentioned risk factors for thromboembolism do not only increase the risk for spontaneous thrombosis, but also the risk for thrombosis during typical high risk situations, such as surgery, trauma of the lower extremities, pregnancy and delivery. APC resistance and antithrombin deficiency increase the risk for development of thrombosis during oral contraceptive intake. Patients, in whom one of the upper mentioned risk factors have been diagnosed, should receive thrombosis prophylaxis during high risk situations. Not all patients with one thromboembolic event and a known risk factor are candidates for long-term oral anticoagulant treatment. Long-term oral anticoagulant treatment should be introduced after exclusion of major contraindications in patients with recurrent events, patients with a combination of risk factors and a life threatening event.