Venous thromboembolism (VTE) is a condition in which a blood clot (a thrombus) forms in a vein and then dislodges to travel in the blood (an embolus). A venous thrombus most commonly occurs in the deep veins of the legs or pelvis; this is then called a deep vein thrombosis (DVT). Blood flow through the affected vein can be limited by the clot, and it can cause swelling and pain in the leg. If it dislodges and travels to the lungs, to the pulmonary arteries, it is called a pulmonary embolism (PE), which in some cases may be fatal. VTE as a term includes both DVT and PE. Major risk factors for VTE include a prior history of DVT, age over 60 years, surgery, obesity, prolonged travel, acute medical illness, cancer, immobility, thrombophilia (an abnormal tendency for the blood to clot) and pregnancy. The diagnosis of VTE is not always straightforward as other conditions may have similar symptoms, thus highlighting the need for guidance on the diagnostic pathways used for the assessment of possible DVT and PE. Failure to diagnose a case of VTE correctly may result in a patient not receiving the correct treatment and potentially suffering a fatal PE as a result. This guideline includes advice on the Wells score, D-dimer measurement, ultrasound and radiological imaging. We have looked at the diagnostic pathways for PE and DVT separately but this guideline did not consider PE risk stratification or the outpatient management of PE as these were beyond our scope. We have focussed on proximal DVT rather than isolated calf vein DVT as the latter is less likely to cause PTS than proximal DVT and also less likely to embolise to the lungs. The current standard practice for the treatment of VTE is anticoagulation. These drugs “thin” the blood and prevent further clotting. There is a wide variation in practice, but patients are usually given a brief course of heparin treatment initially while they start on a 3–6 month course of warfarin. Patients who have had recurrent VTE or who are at high risk of recurrence may be given indefinite treatment with anticoagulants to prevent further VTE episodes. However, anticoagulation treatment is not without risk, for example, the risk of bleeding, and requires the patient to have regular monitoring blood tests. There is a need for guidance about which patients should have such prolonged treatment and how the monitoring should be performed. In addition, there is a wide variation in practice regarding when to test for thrombophilia after VTE and controversy as to how thrombophilia should be managed if it is found on testing. There is also the potential to dissolve the clots using drugs termed thrombolytics which can be achieved both for DVT and PE. Dissolving the clots in the pulmonary arteries may reduce the risk of fatal PE and longer term problems with CTEPH. In the case of DVT, thrombolysis may reduce the risk of fatal PE and PTS. However, the use of thrombolytics may cause side-effects such as bleeding and guidance is needed as to which patients may benefit from their use. This guideline considers the aforementioned in adults (18 years and older) with a suspected or confirmed DVT or PE in primary, secondary and tertiary health-care settings. Within this guideline the following will be considered as special risk groups; people with cancer, people who misuse intravenous drugs, residents of nursing homes, people with physical disabilities who have restricted movement following a VTE and those with learning disabilities who require long-term medication to be taken at home. In particular, people with cancer are at higher risk of developing VTE and may need special advice on how it should be managed, as they may not respond as well when treated with warfarin. Children, people younger than 18 years and pregnant women will not be considered. Prophylaxis against VTE is not addressed as it is already the subject of a NICE clinical guideline (CG92).