Limited accuracy in the clinical diagnosis of deep vein thrombosis (VT) makes such diagnostic tests such as duplex sonography or venography necessary. Exact information on the age and extent of the thrombus are necessary for the clinician to optimize the therapeutic management. The correct diagnosis of calf vein thrombosis and of recurrent VT in patients with postphlebitis changes also has implications for treatment. After exclusion of thrombosis, the radiologist should evaluate the leg for other possible causes of symptoms besides VT. Investigation of the venous system also has a role in the diagnosis in patients with suspected pulmonary embolism. In patients with chronic venous insufficiency the deep venous system should be assessed for patency and venous valve function. The superficial veins should be differentiated in segments with sufficient or insufficient venous valves, and it is also necessary to look for insufficiency of the perforating veins. In patients with superficial phlebitis there is risk of propagation into the deep venous system.