Because clinical signs and symptoms are unreliable the diagnosis of deep vein thrombosis (DVT) should be objectified. Advantages and disadvantages of contrast venography, plethysmography, ultrasound techniques, fibrinogen leg scanning, computer-assisted tomography, magnetic resonance imaging and blood tests are discussed. In patients with a first event of suspected DVT non-invasive methods like serial plethysmography or ultrasound testing are sensitive and specific enough to make a treatment decision. It is safe to withhold anticoagulants if the test remains normal within 1 week. In patients with suspected recurrent DVT new non-invasive techniques are being tested, but up to now the definitive objective diagnostic test continues to be contrast venography. In first period as well as in recurrent DVT D-Dimer testing could be an additional test to exclude active thromboembolism.