Abstract : Venous thromboembolism is the third cardiovascular disease in Europe. The cornerstone of the treatment of deep vein thrombosis is anticoagulation. It aims at avoiding harmful complications : thrombosis extension and recurrence, pulmonary embolism and post-thrombotic syndrome. Due to low molecular weight heparins, and recently, to direct oral anticoagulants, most of the patients can get treatment as outpatients. Unfortunately, despite guideline publications, the management of these patients may be complicated in real life and not correspond to evidence-based medicine. This paper aims at helping the practitian when dealing with this potentially dangerous and often misleading disease. The management of the patient after a 3 to 6-month coagulation treatment will be discussed later in a dedicated paper.